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Free-standing paper-like or foil-like materials are an integral part of our technological society. Their uses include protective layers, chemical filters, components of electrical batteries or supercapacitors, adhesive layers, electronic or optoelectronic components, and molecular storage. Inorganic 'paper-like' materials based on nanoscale components such as exfoliated vermiculite or mica platelets have been intensively studied and commercialized as protective coatings, high-temperature binders, dielectric barriers and gas-impermeable membranes. Carbon-based flexible graphite foils composed of stacked platelets of expanded graphite have long been used in packing and gasketing applications because of their chemical resistivity against most media, superior sealability over a wide temperature range, and impermeability to fluids. The discovery of carbon nanotubes brought about bucky paper, which displays excellent mechanical and electrical properties that make it potentially suitable for fuel cell and structural composite applications. Here we report the preparation and characterization of graphene oxide paper, a free-standing carbon-based membrane material made by flow-directed assembly of individual graphene oxide sheets. This new material outperforms many other paper-like materials in stiffness and strength. Its combination of macroscopic flexibility and stiffness is a result of a unique interlocking-tile arrangement of the nanoscale graphene oxide sheets.

Dionaea muscipula belongs to a small group of carnivorous plants which have movable leaf traps1. Triggering an inner leaf hair causes the trap to close and this is accompanied by a characteristic action potential2. During histological observations we noted that in mature leaves there were large concentrations of a red pigment in the digestive glands, the inner layer of the leaf cells, and particularly in the cells at the base of the sensitive trigger hair. Presuming this pigment to be an anthocyanin3, we analysed it by column chromatography4, paper chromatography, ultra-violet and infra-red absorption spectra.

Grain boundaries in graphene are formed by the joining of islands during the initial growth stage, and these boundaries govern transport properties and related device performance. Although information on the atomic rearrangement at graphene grain boundaries can be obtained using transmission electron microscopy and scanning tunnelling microscopy, large-scale information regarding the distribution of graphene grain boundaries is not easily accessible. Here we use optical microscopy to observe the grain boundaries of large-area graphene (grown on copper foil) directly, without transfer of the graphene. This imaging technique was realized by selectively oxidizing the underlying copper foil through graphene grain boundaries functionalized with O and OH radicals generated by ultraviolet irradiation under moisture-rich ambient conditions: selective diffusion of oxygen radicals through OH-functionalized defect sites was demonstrated by density functional calculations. The sheet resistance of large-area graphene decreased as the graphene grain sizes increased, but no strong correlation with the grain size of the copper was revealed, in contrast to a previous report. Furthermore, the influence of graphene grain boundaries on crack propagation (initialized by bending) and termination was clearly visualized using our technique. Our approach can be used as a simple protocol for evaluating the grain boundaries of other two-dimensional layered structures, such as boron nitride and exfoliated clays.

MARTIN and co-workers1 extended their technique of partition chromatography of the amino-acids to the micro-scale by using filter paper instead of silica gel as the support for the stationary phase (water). We have found that the modified partition chromatogram for penicillin using buffer as the stationary phase2 can also be adapted to the micro-scale by the same change in support. Colour reactions are not readily applicable to locate the positions of the invisible developed zones, so that a microbiological procedure has been used both to identify and to determine approximately the types and amounts of penicillins resolved on the buffered paper strip. The procedure is as follows.

Increasing energy costs and environmental concerns have emphasized the need to produce sustainable renewable fuels and chemicals. Major efforts to this end are focused on the microbial production of high-energy fuels by cost-effective 'consolidated bioprocesses'. Fatty acids are composed of long alkyl chains and represent nature's 'petroleum', being a primary metabolite used by cells for both chemical and energy storage functions. These energy-rich molecules are today isolated from plant and animal oils for a diverse set of products ranging from fuels to oleochemicals. A more scalable, controllable and economic route to this important class of chemicals would be through the microbial conversion of renewable feedstocks, such as biomass-derived carbohydrates. Here we demonstrate the engineering of Escherichia coli to produce structurally tailored fatty esters (biodiesel), fatty alcohols, and waxes directly from simple sugars. Furthermore, we show engineering of the biodiesel-producing cells to express hemicellulases, a step towards producing these compounds directly from hemicellulose, a major component of plant-derived biomass.

The ability to deposit and tailor reliable semiconducting films (with a particular recent emphasis on ultrathin systems) is indispensable for contemporary solid-state electronics. The search for thin-film semiconductors that provide simultaneously high carrier mobility and convenient solution-based deposition is also an important research direction, with the resulting expectations of new technologies (such as flexible or wearable computers, large-area high-resolution displays and electronic paper) and lower-cost device fabrication. Here we demonstrate a technique for spin coating ultrathin (approximately 50 A), crystalline and continuous metal chalcogenide films, based on the low-temperature decomposition of highly soluble hydrazinium precursors. We fabricate thin-film field-effect transistors (TFTs) based on semiconducting SnS(2-x)Se(x) films, which exhibit n-type transport, large current densities (>10(5) A cm(-2)) and mobilities greater than 10 cm2 V(-1) s(-1)--an order of magnitude higher than previously reported values for spin-coated semiconductors. The spin-coating technique is expected to be applicable to a range of metal chalcogenides, particularly those based on main group metals, as well as for the fabrication of a variety of thin-film-based devices (for example, solar cells, thermoelectrics and memory devices).

AMINOPEPTIDASE B is a mammalian enzyme selective for the N-terminal arginine and lysine residues of peptides. This enzyme has been described, purified and characterized1–3. It occurs in many of the organs of the rat and other species and, like cathepsin C, is known to be activated markedly by chloride ions. Aminopeptidase B is almost inactive in the complete absence of these ions and shows maximal activity at physiological concentrations of chloride. The physiological role of the enzyme was unknown, and an effort was made to determine whether some naturally occurring N-terminal arginine and lysine peptides could be split by the enzyme. A peptide of this type, kallidin-10, is liberated from the serum α2-globulin fraction by proteolytic enzymes such as trypsin and kallikrein. Bradykinin is formed when the N-terminal lysine of kallidin-10 is released. Bradykinin has an N-terminal arginine adjacent to proline. Our results show that kallidin-10 is transformed to bradykinin by aminopeptidase B without further degradation (liberation of arginine) of the nonapeptide.

IT is well known to analysts that starch is very far from being an ideal indicator for iodine, and the disadvantages from which it suffers are not all avoided by the use of an acid-solubilized starch. It will be agreed that the chief disadvantages attending the use of starch are : (1) the insolubility of starch in cold water ; (ii) the instability of starch dispersions in water, in consequence of which a stock solution soon deposits a flocculent precipitate of retrograded starch ; (iii) that starch gives with iodine a water-insoluble complex, the formation of which precludes the addition of the indicator early in the titration ; and (iv) the “drift” of end-point which is particularly marked when the solutions used are dilute.

The remarkable mechanical properties of carbon nanotubes arise from the exceptional strength and stiffness of the atomically thin carbon sheets (graphene) from which they are formed. In contrast, bulk graphite, a polycrystalline material, has low fracture strength and tends to suffer failure either by delamination of graphene sheets or at grain boundaries between the crystals. Now Stankovich et al. have produced an inexpensive polymer-matrix composite by separating graphene sheets from graphite and chemically tuning them. The material contains dispersed graphene sheets and offers access to a broad range of useful thermal, electrical and mechanical properties. Individual sheets of graphene can be readily incorporated into a polymer matrix, giving rise to composite materials having potentially useful electronic properties. Graphene sheets—one-atom-thick two-dimensional layers of sp2-bonded carbon—are predicted to have a range of unusual properties. Their thermal conductivity and mechanical stiffness may rival the remarkable in-plane values for graphite (∼3,000 W m-1 K-1 and 1,060 GPa, respectively); their fracture strength should be comparable to that of carbon nanotubes for similar types of defects1,2,3; and recent studies have shown that individual graphene sheets have extraordinary electronic transport properties4,5,6,7,8. One possible route to harnessing these properties for applications would be to incorporate graphene sheets in a composite material. The manufacturing of such composites requires not only that graphene sheets be produced on a sufficient scale but that they also be incorporated, and homogeneously distributed, into various matrices. Graphite, inexpensive and available in large quantity, unfortunately does not readily exfoliate to yield individual graphene sheets. Here we present a general approach for the preparation of graphene-polymer composites via complete exfoliation of graphite9 and molecular-level dispersion of individual, chemically modified graphene sheets within polymer hosts. A polystyrene–graphene composite formed by this route exhibits a percolation threshold10 of ∼0.1 volume per cent for room-temperature electrical conductivity, the lowest reported value for any carbon-based composite except for those involving carbon nanotubes11; at only 1 volume per cent, this composite has a conductivity of ∼0.1 S m-1, sufficient for many electrical applications12. Our bottom-up chemical approach of tuning the graphene sheet properties provides a path to a broad new class of graphene-based materials and their use in a variety of applications.

Mixing dried human sewage sludge with pulverized coal is being evaluated as a way to reduce carbon dioxide emissions from traditionally coal-fired power plants, as well as to overcome sewage-disposal problems. Here we investigate the effects of inhaling particles emitted from the combustion of this mixture and find that these cause significantly more lung damage in mice than do particles from coal alone, probably because of their zinc content. Our findings indicate that the use of dried municipal sewage sludge as a 'green' (CO2-neutral) replacement fuel should be considered with caution.

The third of four weekly articles looks at how the financial crisis is slowing efforts to commercialize next-generation ethanol.

Bottled mineral water, says Daedalus, would have added scientific credibility and true health value with the addition of minerals such as calcium and magnesium, and vitamin C. He plans to market the resulting product as ‘Reinforced Water’.

Unmethylated reovirus and VSV mRNAs are specifically methylated to form 5'-terminal structures of the type, m-7-G(5')ppp(5')N by protein synthesising extracts prepared from wheat germ and mouse L cells. Reticulocyte mRNA also contains 5'-terminal m-7-G. MRNAs having 5'-terminal m-7-G stimulate protein synthesis in vitro. Removal of m-7-G by beta-elimination abolishes translation of the mRNAs.

The mRNA synthesised in vitro by the virion-associated RNA polymerase of vesicular stomatitis virus contains the novel 5′-terminal structure G(5′)ppp(5′)Ap....

The large amounts of microalgae produced in waste treatment ponds during sewage purification are a potential source of methane and fertiliser. If techniques can be developed for the selective cultivation of filamentous or colonial microalgae, harvesting of microalgal biomass could be accomplished by low-cost straining or sedimentation methods.

Attempts have been made to find green plants which produce low-molecular weight hydrocarbons1,2 and to find seaweeds which produce hydrogen from water utilizing solar energy3,4. These attempts are aimed at finding methods of making use of the photosynthetic process in plants for direct production. Most green plants, however, synthesize carbohydrates, such as sugar, starch and/or cellulose, from water and carbon dioxide. The C4 plants5, such as corns and sugar cane, grow rapidly, utilizing solar energy with ∼1% efficiency for the fixation of CO2. This value is 10 times larger than that of the average efficiency of photosynthesis of plants, 0.1%. However, the carbohydrates produced by these plants cannot be used directly as fuel. Here we show a new route for the conversion of carbohydrates into hydrogen (a clean fuel in the hydrogen energy system), taking advantage of the photocatalytic process. We found that the irradiation of the carbohydrates, not only sugar or starch but also cellulose, in the presence of water and a RuO2/TiO2/Pt photocatalyst powder leads to the efficient production of hydrogen gas.

SEVERAL workers1–3 have attempted to retrieve palaeoclimatic information from the 12C/13C ratios in tree rings. Their use of whole wood, however, has been unsatisfactory, in that the extractives are isotopically much lighter than cellulose4 and vary considerably in content between heartwood and sapwood. Further, lignin is usually ∼ 3‰ lighter than cellulose and it is well known from the extensive literature on the chemistry of wood that the lignin-to-cellulose ratio varies not only across one ring, that is, in late wood as compared to early wood, but up and down and radially across the tree5. The science of dendroclimatology is based on the observation that narrow rings represent a cold (or dry) year. In most conifers the amount of late wood remains relatively constant and it is the thickness of early wood that varies. A narrow ring would be expected to have a smaller proportion of early wood than a wide ring. The different proportions of lignin in the two rings would then lead to a different isotopic ratio for the whole wood. If whole wood is used the δ13C measurements will reflect the relative proportions of lignin and cellulose. We therefore separated the lignin and cellulose from the wood before making δ13C determinations, hoping that the temperature coefficients of their production reactions might be large enough to enable the temperatures at which the carbon was fixed from the atmosphere to be measured. We also wished to investigate the possibility that the difference in δ13C between the cellulose and lignin might reflect temperature.

METHYLATION of a specific nucleotide occurs at the initial stage of transcription of the double-stranded RNA genome in cytoplasmic polyhedrosis virus (CPV) in the presence of the methyl-group donor, S-adenosyl methionine (SAM)1. Methylation of mRNA has been reported recently not only for other viruses2,3 but also for a mouse L-cell4. Since methylation seems to be coupled with the initiation of mRNA synthesis in CPV, the structure of the initiation (5′) terminus of mRNA transcribed from CPV by the virus-containing RNA polymerase is of particular interest.

The influenza virus polymerase transcribes or replicates the segmented RNA genome (viral RNA) into viral messenger RNA or full-length copies. To initiate RNA synthesis, the polymerase binds to the conserved 3′ and 5′ extremities of the viral RNA. Here we present the crystal structure of the heterotrimeric bat influenza A polymerase, comprising subunits PA, PB1 and PB2, bound to its viral RNA promoter. PB1 contains a canonical RNA polymerase fold that is stabilized by large interfaces with PA and PB2. The PA endonuclease and the PB2 cap-binding domain, involved in transcription by cap-snatching, form protrusions facing each other across a solvent channel. The 5′ extremity of the promoter folds into a compact hook that is bound in a pocket formed by PB1 and PA close to the polymerase active site. This structure lays the basis for an atomic-level mechanistic understanding of the many functions of influenza polymerase, and opens new opportunities for anti-influenza drug design. The crystal structure of the bat-specific influenza A polymerase in complex with the viral RNA promoter is presented, revealing how binding of the 5′ end of the viral RNA is required to activate or enhance the polymerase allosterically. Stephen Cusack and colleagues have solved the crystal structure of the complete influenza polymerase, comprising subunits PA, PB1 and PB2, bound to its viral RNA promoter. In the first of two papers they present the structure of the polymerase from a bat-specific influenza A virus, which is evolutionarily close to human/avian influenza A strains. The second paper presents the structure of the polymerase from a human isolate of influenza B. Together, the structures provide a wealth of information about how the influenza polymerase functions and how the different subunits interact with each other.

The search for safe, non-addictive versions of morphine and other opioid drugs has just received a boost with the solving of the crystal structures of the receptors to which the drugs bind. See Articles p.321 & p.327 , Letters p.395 & p.400 Four papers in this issue of Nature present the long-awaited high-resolution crystal structures of the four known opioid receptors in ligand-bound conformations. These G-protein-coupled receptors are the targets of a broad range of drugs, including painkillers, antidepressants, anti-anxiety agents and anti-addiction medications. Brian Kobilka’s group reports the crystal structure of the µ-opioid receptor bound to a morphinan antagonist and the δ-opioid receptor bound to naltrindole. Raymond Stevens’ group reports on the κ-opioid receptor bound to the selective antagonist JDTic, and the nociceptin/orphanin FQ receptor bound to a peptide mimetic. In an associated News and Views, Marta Filizola and Lakshmi Devi discuss the implications of these landmark papers for research on the mechanisms underlying receptor function and drug development.

The modified DNA base 5-hydroxymethylcytosine (5hmC), sometimes called the sixth base, is present in the mammalian genome where it is generated by oxidation of 5-methylcytosine (5mC; the fifth base) by enzymes of the Tet family. Four papers in this issue, from the Helin, Zhang, Rao and Reik laboratories, respectively, report on the genome-wide distribution of Tet1 and/or 5hmC in mouse embryonic stem cells using the ChIP-seq technique. Links between Tet1 and transcription regulation — both activation and repression — are revealed. Anjana Rao and colleagues also describe two alternative methods with increased sensitivity for mapping single 5hmC bases. In the associated News & Views, Nathalie Veron and Antoine H. F. M. Peters discuss what these and other recent papers reveal about the role of Tet proteins in regulating DNA methylation and gene expression. Epigenetic modification of the mammalian genome by DNA methylation (5-methylcytosine) has a profound impact on chromatin structure, gene expression and maintenance of cellular identity1. The recent demonstration that members of the Ten-eleven translocation (Tet) family of proteins can convert 5-methylcytosine to 5-hydroxymethylcytosine raised the possibility that Tet proteins are capable of establishing a distinct epigenetic state2,3. We have recently demonstrated that Tet1 is specifically expressed in murine embryonic stem (ES) cells and is required for ES cell maintenance2. Using chromatin immunoprecipitation coupled with high-throughput DNA sequencing, here we show in mouse ES cells that Tet1 is preferentially bound to CpG-rich sequences at promoters of both transcriptionally active and Polycomb-repressed genes. Despite an increase in levels of DNA methylation at many Tet1-binding sites, Tet1 depletion does not lead to downregulation of all the Tet1 targets. Interestingly, although Tet1-mediated promoter hypomethylation is required for maintaining the expression of a group of transcriptionally active genes, it is also involved in repression of Polycomb-targeted developmental regulators. Tet1 contributes to silencing of this group of genes by facilitating recruitment of PRC2 to CpG-rich gene promoters. Thus, our study not only establishes a role for Tet1 in modulating DNA methylation levels at CpG-rich promoters, but also reveals a dual function of Tet1 in promoting transcription of pluripotency factors as well as participating in the repression of Polycomb-targeted developmental regulators.

The mechanism of substrate loading in multisubunit RNA polymerase is crucial for understanding the general principles of transcription yet remains hotly debated. Here we report the 3.0-A resolution structures of the Thermus thermophilus elongation complex (EC) with a non-hydrolysable substrate analogue, adenosine-5′-[(α,β)-methyleno]-triphosphate (AMPcPP), and with AMPcPP plus the inhibitor streptolydigin. In the EC/AMPcPP structure, the substrate binds to the active (‘insertion’) site closed through refolding of the trigger loop (TL) into two α-helices. In contrast, the EC/AMPcPP/streptolydigin structure reveals an inactive (‘preinsertion’) substrate configuration stabilized by streptolydigin-induced displacement of the TL. Our structural and biochemical data suggest that refolding of the TL is vital for catalysis and have three main implications. First, despite differences in the details, the two-step preinsertion/insertion mechanism of substrate loading may be universal for all RNA polymerases. Second, freezing of the preinsertion state is an attractive target for the design of novel antibiotics. Last, the TL emerges as a prominent target whose refolding can be modulated by regulatory factors. Two complementary papers this week focus on the structure and function of bacterial RNA polymerase. In the first, the enzyme is bound to the DNA template and RNA product, to give a close-up of the transcription elongation complex. The structure reveals details of the DNA-to-RNA transcription process, vital to all living cells. In the second paper, the RNA polymerase elongation complex is pictured bound to various substrate analogues and to an antibiotic, revealing the mechanism of substrate loading and antibiotic inhibition. Comparisons between the structures of human and bacteria RNA polymerase should aid in drug design: RNA polymerase is a target of antibiotics, including rifamycin and its derivatives. Crystal structures of bacterial RNA polymerase elongation complexes bound to NTP substrate analogues with an antibiotic, revealing the mechanism of substrate loading and antibiotic inhibition.

Lignin is a heterogeneous aromatic biopolymer that accounts for nearly 30% of the organic carbon on Earth and is one of the few renewable sources of aromatic chemicals. As the most recalcitrant of the three components of lignocellulosic biomass (cellulose, hemicellulose and lignin), lignin has been treated as a waste product in the pulp and paper industry, where it is burned to supply energy and recover pulping chemicals in the operation of paper mills. Extraction of higher value from lignin is increasingly recognized as being crucial to the economic viability of integrated biorefineries. Depolymerization is an important starting point for many lignin valorization strategies, because it could generate valuable aromatic chemicals and/or provide a source of low-molecular-mass feedstocks suitable for downstream processing. Commercial precedents show that certain types of lignin (lignosulphonates) may be converted into vanillin and other marketable products, but new technologies are needed to enhance the lignin value chain. The complex, irregular structure of lignin complicates chemical conversion efforts, and known depolymerization methods typically afford ill-defined products in low yields (that is, less than 10-20wt%). Here we describe a method for the depolymerization of oxidized lignin under mild conditions in aqueous formic acid that results in more than 60wt% yield of low-molecular-mass aromatics. We present the discovery of this facile C-O cleavage method, its application to aspen lignin depolymerization, and mechanistic insights into the reaction. The broader implications of these results for lignin conversion and biomass refining are also considered.

In a study of creatinine metabolism in the rat, to be published elsewhere, creatinine has been identified qualitatively by means of the technique of partition chromatography on paper strips developed by Consden, Gordon and Martin1. The apparatus described by Dent2 has been used, with the further refinement of accurate temperature control obtained by immersing the outer glass container in a water thermostat at 15 ± 0·05°C. Solutions containing c. 20–30 γ of creatinine were pipetted on to the paper strips, and after treatment with an appropriate solvent, for example, collidine for 24 hours, the creatinine bands were located by spraying with alkaline sodium picrate.

Structures of the heat-sensitive TRPV1 ion channel have been solved using single-particle electron cryo-microscopy, representing a landmark in the use of this technique for structural biology. See Articles p.107 & p.113 Transient receptor potential (TRP) channels are sensors for a wide range of physical and chemical stimuli. In the first of a pair of related papers, Maofu Liao et al. solve the high-resolution electron cryo-microscopy structure of rat TRPV1, the receptor for capsaicin (a pungent agent from chili peppers), in a 'closed' state. The overall structure is fairly similar to that of a voltage-gated ion channel, but there are several structural features unique to TRP channels. In the second paper, Erhu Cao et al. present the structures of rat TRPV1 in the presence of a peptide neurotoxin (resiniferatoxin) and in the presence of capsaicin, yielding structures of activated states of the channel. Comparison of the closed and open structures suggests that TRPV1 has a unique two-gate mechanism of channel activation.

In prokaryotes and eukaryotes many gene activators work synergistically. For example, two dimers of lambda repressor interact to promote binding of these proteins to DNA, a reaction that is crucial at the repressor concentrations found in lysogens. In this case one of the bound dimers activates transcription, evidently by touching RNA polymerase. In another example, the yeast transcriptional activator GAL4, which can stimulate transcription in many eukaryotes, binds to multiple sites on DNA to activate transcription synergistically; the presence of two such sites can elicit a level of transcription more than twice that found with a single site. In this paper we show that synergistic activation by each of several GAL4 derivatives involves a mechanism different from that illustrated by the lambda repressor: multiple activator molecules can work synergistically under conditions in which their binding sites on DNA are saturated. The accompanying paper shows that under similar conditions of activator excess, GAL4 derivatives work synergistically with a heterologous mammalian gene activator. These results support the idea that eukaryotic activators can cooperate not by directly interacting but by simultaneously touching some component(s) of the transcriptional machinery.

The Huygens probe landed on Titan on 14 January this year, and seven papers published in this issue record the encounter. They describe a world that resembles a primitive Earth, complete with weather systems and geological activity. The ‘Huygens on Titan’ section opens with an overview of the descent and landing and a News and Views piece. Tomasko et al. describe the dry riverbed and drainage channels seen during Huygens' descent, evidence that liquid methane falls as rain or erupts from cryovolcanoes, periodically flooding the surface. This paper includes the images used on the cover to the Huygens section. Niemann et al. measured the abundances of isotopes of argon, nitrogen and carbon in the atmosphere, and conclude that there is no evidence that Titan's methane comes from biological activity. Fulchignoni et al. obtained precise measurements of temperature and pressure from the upper atmosphere right down to the surface. On the way down Huygens recorded evidence for lightning. Zarnecki et al. report that the probe landed on a relatively smooth surface of icy grains with the consistency of wet clay or sand. Isral et al. report that the aerosols in Titan's clouds have solid cores made from complex organic molecules containing carbon and nitrogen. And Bird et al. found that on average Titan's winds blow in the same direction as the moon rotates, and that close to the surface these winds are very weak, travelling at around walking speed. One of Titan's most intriguing attributes is its copious but featureless atmosphere. The Voyager 1 fly-by and occultation in 1980 provided the first radial survey of Titan's atmospheric pressure and temperature1,2 and evidence for the presence of strong zonal winds3. It was realized that the motion of an atmospheric probe could be used to study the winds, which led to the inclusion of the Doppler Wind Experiment4 on the Huygens probe5. Here we report a high resolution vertical profile of Titan's winds, with an estimated accuracy of better than 1 m s-1. The zonal winds were prograde during most of the atmospheric descent, providing in situ confirmation of superrotation on Titan. A layer with surprisingly slow wind, where the velocity decreased to near zero, was detected at altitudes between 60 and 100 km. Generally weak winds (∼1 m s-1) were seen in the lowest 5 km of descent.

Tet proteins mediate the hydroxymethylation of DNA. New work reveals their function in gene regulation and the extent of their activity throughout the genome of embryonic stem cells. See Article p.343 & Letters p.389 , p.394 & p.398 The modified DNA base 5-hydroxymethylcytosine (5hmC), sometimes called the sixth base, is present in the mammalian genome where it is generated by oxidation of 5-methylcytosine (5mC; the fifth base) by enzymes of the Tet family. Four papers in this issue, from the Helin, Zhang, Rao and Reik laboratories, respectively, report on the genome-wide distribution of Tet1 and/or 5hmC in mouse embryonic stem cells using the ChIP-seq technique. Links between Tet1 and transcription regulation — both activation and repression — are revealed. Anjana Rao and colleagues also describe two alternative methods with increased sensitivity for mapping single 5hmC bases. In the associated News & Views, Nathalie Veron and Antoine H. F. M. Peters discuss what these and other recent papers reveal about the role of Tet proteins in regulating DNA methylation and gene expression.

The first analyses of data sent by the Huygens probe from Saturn's largest moon Titan are flooding in. They paint a picture of a ‘Peter Pan’ world — potentially like Earth, but with its development frozen at an early stage. The Huygens probe landed on Titan on 14 January this year, and seven papers published in this issue record the encounter. They describe a world that resembles a primitive Earth, complete with weather systems and geological activity. The ‘Huygens on Titan’ section opens with an overview of the descent and landing and a News and Views piece. Tomasko et al. describe the dry riverbed and drainage channels seen during Huygens' descent, evidence that liquid methane falls as rain or erupts from cryovolcanoes, periodically flooding the surface. This paper includes the images used on the cover to the Huygens section. Niemann et al. measured the abundances of isotopes of argon, nitrogen and carbon in the atmosphere, and conclude that there is no evidence that Titan's methane comes from biological activity. Fulchignoni et al. obtained precise measurements of temperature and pressure from the upper atmosphere right down to the surface. On the way down Huygens recorded evidence for lightning. Zarnecki et al. report that the probe landed on a relatively smooth surface of icy grains with the consistency of wet clay or sand. Isral et al. report that the aerosols in Titan's clouds have solid cores made from complex organic molecules containing carbon and nitrogen. And Bird et al. found that on average Titan's winds blow in the same direction as the moon rotates, and that close to the surface these winds are very weak, travelling at around walking speed.

Nature 437, 376–380 (2005) In this paper, the second name of Chun Heen Ho was misspelled as He. In the earlier Corrigendum (Nature 439, 502; 2006) the surnames of Stephen K. Hutchison and Brian C. Godwin were misspelled as Hutchinson and Goodwin. In addition, in the Supplementary Information of the original paper, there were errors in the sequences of the DNA capture and enrichment primers, and in the formulation of emulsion oil.

Nature 403, 408–410 (2000). Several papers were recently the subject of an independent investigation conducted at the behest of Bell Laboratories, Lucent Technologies. The independent committee reviewed concerns related to the validity of data associated with the device measurements described in those papers.

The pattern of volatile element depletion in the silicate Earth suggests that melting and vaporization on precursor bodies and during accretion were responsible for the volatile element contents of Earth. In comparison to primitive, chondritic meteorites, which are widely thought to be the building blocks of Earth, Earth and other differentiated planetary bodies are chemically fractionated, with Earth's crust and mantle—the 'silicate Earth'—being strongly depleted in moderately volatile elements (such as lead, zinc, indium and the alkali metals). Two papers in this week's issue suggest that this difference in composition between chondritic meteorites and Earth could be a natural consequence of vapour loss from magma on the surface of growing planetesimals. Ashley Norris and Bernard Wood examined the melting processes that would have occurred during accretion on Earth and its precursor bodies and performed vaporization experiments under conditions of fixed temperature and oxygen partial pressure. They found that the pattern of volatile-element depletion in the silicate Earth is consistent with partial melting and vaporization rather than with simple accretion of a volatile-rich chondrite-like body. Remco Hin and co-authors show that differentiated planetary bodies have isotopically heavier magnesium compositions compared to chondritic meteorites, and conclude that this could be due to the isotopic fractionation between liquid and vapour, followed by vapour escape during accretionary growth of planetesimals. The silicate Earth is strongly depleted in moderately volatile elements (such as lead, zinc, indium and alkali elements) relative to CI chondrites, the meteorites that compositionally most closely resemble the Sun1. This depletion may be explained qualitatively by accretion of 10 to 20 per cent of a volatile-rich body to a reduced volatile-free proto-Earth2,3, followed by partial extraction of some elements to the core1. However, there are several unanswered questions regarding the sources of Earth’s volatiles4,5, notably the overabundance of indium in the silicate Earth. Here we examine the melting processes that occurred during accretion on Earth and precursor bodies and report vaporization experiments under conditions of fixed temperature and oxygen fugacity. We find that the pattern of volatile element depletion in the silicate Earth is consistent with partial melting and vaporization rather than with simple accretion of a volatile-rich chondrite-like body. We argue that melting and vaporization on precursor bodies and possibly during the giant Moon-forming impact6,7,8 were responsible for establishing the observed abundances of moderately volatile elements in Earth.

Observations from a Greenland ice core reveal that tropospheric oxidants are sensitive to climate-driven changes in reactive halogen chemistry and stratosphere-to-troposphere transport of ozone, in addition to ozone precursor emissions. The abundance of tropospheric oxidants, such as ozone (O3) and the hydroxyl radical (OH), determines the lifetimes of reduced trace gases such as methane, but the response of tropospheric oxidants to climate change is poorly constrained. This paper reports isotopic data from a Greenland ice core covering the most recent glacial–interglacial cycle that demonstrate the sensitivity of tropospheric oxidants to climate change. The authors observe an increase in the O3/HOx ratio in cold climates, the opposite of current expectations, and suggest that this increase might be driven by enhanced reactive halogen chemistry, enhanced stratosphere-to-troposphere transport of O3, or a combination of the two. These processes require further consideration when assessing global methane variations during periods of abrupt climate change. The abundance of tropospheric oxidants, such as ozone (O3) and hydroxyl (OH) and peroxy radicals (HO2 + RO2), determines the lifetimes of reduced trace gases such as methane and the production of particulate matter important for climate and human health. The response of tropospheric oxidants to climate change is poorly constrained owing to large uncertainties in the degree to which processes that influence oxidants may change with climate1 and owing to a lack of palaeo-records with which to constrain levels of atmospheric oxidants during past climate transitions2. At present, it is thought that temperature-dependent emissions of tropospheric O3 precursors and water vapour abundance determine the climate response of oxidants, resulting in lower tropospheric O3 in cold climates while HOx (= OH + HO2 + RO2) remains relatively buffered3. Here we report observations of oxygen-17 excess of nitrate (a proxy for the relative abundance of atmospheric O3 and HOx) from a Greenland ice core over the most recent glacial–interglacial cycle and for two Dansgaard–Oeschger events. We find that tropospheric oxidants are sensitive to climate change with an increase in the O3/HOx ratio in cold climates, the opposite of current expectations. We hypothesize that the observed increase in O3/HOx in cold climates is driven by enhanced stratosphere-to-troposphere transport of O3, and that reactive halogen chemistry is also enhanced in cold climates. Reactive halogens influence the oxidative capacity of the troposphere directly as oxidants themselves and indirectly4 via their influence on O3 and HOx. The strength of stratosphere-to-troposphere transport is largely controlled by the Brewer–Dobson circulation5, which may be enhanced in colder climates owing to a stronger meridional gradient of sea surface temperatures6, with implications for the response of tropospheric oxidants7 and stratospheric thermal and mass balance8. These two processes may represent important, yet relatively unexplored, climate feedback mechanisms during major climate transitions.

Lithium–oxygen batteries allow oxygen to be reduced at the battery’s cathode when a current is drawn; in present-day batteries, this results in formation of Li2O2, but it is now shown that another high energy density material, namely LiO2, with better electronic conduction can be used instead as the discharge product, if the electrode is decorated with iridium nanoparticles. Nonaqueous lithium–air batteries have a much superior theoretical gravimetric energy density compared to conventional lithium ion batteries, and thus have the potential for making long-range electric vehicles a reality. Batteries based on sodium and potassium superoxides have recently been reported, but thermodynamically unstable lithium superoxide (LiO2), with its potential high energy density, has proved more problematic. This paper demonstrates that crystalline LiO2 can be stabilized in a Li–O2 battery by using a suitable cathode material — reduced graphene oxide decorated with iridium nanoparticles. A battery based on this new lithium–oxygen chemistry was demonstrated through 40 cycles before failure, achieving high efficiency and good capacity. Batteries based on sodium superoxide and on potassium superoxide have recently been reported1,2,3. However, there have been no reports of a battery based on lithium superoxide (LiO2), despite much research4,5,6,7,8 into the lithium–oxygen (Li–O2) battery because of its potential high energy density. Several studies9,10,11,12,13,14,15,16 of Li–O2 batteries have found evidence of LiO2 being formed as one component of the discharge product along with lithium peroxide (Li2O2). In addition, theoretical calculations have indicated that some forms of LiO2 may have a long lifetime17. These studies also suggest that it might be possible to form LiO2 alone for use in a battery. However, solid LiO2 has been difficult to synthesize in pure form18 because it is thermodynamically unstable with respect to disproportionation, giving Li2O2 (refs 19, 20). Here we show that crystalline LiO2 can be stabilized in a Li–O2 battery by using a suitable graphene-based cathode. Various characterization techniques reveal no evidence for the presence of Li2O2. A novel templating growth mechanism involving the use of iridium nanoparticles on the cathode surface may be responsible for the growth of crystalline LiO2. Our results demonstrate that the LiO2 formed in the Li–O2 battery is stable enough for the battery to be repeatedly charged and discharged with a very low charge potential (about 3.2 volts). We anticipate that this discovery will lead to methods of synthesizing and stabilizing LiO2, which could open the way to high-energy-density batteries based on LiO2 as well as to other possible uses of this compound, such as oxygen storage.

Precise measurements of the tungsten isotopic composition of lunar rocks show that the Moon exhibits a well-resolved excess of 182W of about 27 parts per million over the present-day Earth’s mantle: this excess is consistent with the expected 182W difference resulting from a late veneer with a total mass and composition inferred from previously measured highly siderophile elements. Two papers published in this issue of Nature present precise measurements of tungsten isotope composition in lunar rocks that are best explained by the Earth and Moon having had similar composition immediately following formation of the Moon, and then having diverged as a result of disproportional late accretion of material to the two bodies. Mathieu Touboul et al. found small 182W excess of about 21 parts per million relative to the present-day Earth's mantle in metals extracted from two KREEP-rich Apollo 16 impact-melt rocks, while Thomas Kruijer et al. measured tungsten isotopes in seven KREEP-rich whole rock samples that span a wide range of cosmic ray exposure ages, and found a 182W excess of about 27 parts per million over the present-day Earth's mantle. According to the most widely accepted theory of lunar origin, a giant impact on the Earth led to the formation of the Moon, and also initiated the final stage of the formation of the Earth’s core1. Core formation should have removed the highly siderophile elements (HSE) from Earth’s primitive mantle (that is, the bulk silicate Earth), yet HSE abundances are higher than expected2. One explanation for this overabundance is that a ‘late veneer’ of primitive material was added to the bulk silicate Earth after the core formed2. To test this hypothesis, tungsten isotopes are useful for two reasons: first, because the late veneer material had a different 182W/184W ratio to that of the bulk silicate Earth, and second, proportionally more material was added to the Earth than to the Moon3. Thus, if a late veneer did occur, the bulk silicate Earth and the Moon must have different 182W/184W ratios. Moreover, the Moon-forming impact would also have created 182W differences because the mantle and core material of the impactor with distinct 182W/184W would have mixed with the proto-Earth during the giant impact. However the 182W/184W of the Moon has not been determined precisely enough to identify signatures of a late veneer or the giant impact. Here, using more-precise measurement techniques, we show that the Moon exhibits a 182W excess of 27 ± 4 parts per million over the present-day bulk silicate Earth. This excess is consistent with the expected 182W difference resulting from a late veneer with a total mass and composition inferred from HSE systematics2. Thus, our data independently show that HSE abundances in the bulk silicate Earth were established after the giant impact and core formation, as predicted by the late veneer hypothesis. But, unexpectedly, we find that before the late veneer, no 182W anomaly existed between the bulk silicate Earth and the Moon, even though one should have arisen through the giant impact. The origin of the homogeneous 182W of the pre-late-veneer bulk silicate Earth and the Moon is enigmatic and constitutes a challenge to current models of lunar origin.

Polyketide natural products constitute a broad class of compounds with diverse structural features and biological activities. Their biosynthetic machinery, represented by type I polyketide synthases (PKSs), has an architecture in which successive modules catalyse two-carbon linear extensions and keto-group processing reactions on intermediates covalently tethered to carrier domains. Here we used electron cryo-microscopy to determine sub-nanometre-resolution three-dimensional reconstructions of a full-length PKS module from the bacterium Streptomyces venezuelae that revealed an unexpectedly different architecture compared to the homologous dimeric mammalian fatty acid synthase. A single reaction chamber provides access to all catalytic sites for the intramodule carrier domain. In contrast, the carrier from the preceding module uses a separate entrance outside the reaction chamber to deliver the upstream polyketide intermediate for subsequent extension and modification. This study reveals for the first time, to our knowledge, the structural basis for both intramodule and intermodule substrate transfer in polyketide synthases, and establishes a new model for molecular dissection of these multifunctional enzyme systems. Polyketide synthases are multidomain enzymes that produce polyketides, which form the basis of many therapeutic agents; here, electron cryo-microscopy is used to establish the structure of a bacterial full-length module, and to elucidate the structural basis of both intramodule and intermodule substrate transfer. Polyketide synthases (PKSs) are multi-domain enzyme complexes producing polyketides, a large class of secondary metabolites — in other words, natural products. Two papers from Georgios Skiniotis and colleagues use cryo-electron microscopy to probe the structure of an intact module of a full-length multienzyme PKS module involved in pikromycin biosynthesis in the bacterium Streptomyces venezuelae in different functional states. The structures reveal how the ketosynthase, acyltransferase, ketoreductase and acyl carrier protein (ACP) domains interact during the catalytic cycle. In each state the ACP is differentially positioned to facilitate intermediate transfer for the next catalytic step and for transfer to the next module.

Polyketide synthases (PKSs) are multidomain enzymes that produce polyketides, which form the basis of many therapeutic agents; here, electron cryo-microscopy is used to probe the structure of an intact module of a multi-enzyme PKS in different functional states. Polyketide synthases (PKSs) are multi-domain enzyme complexes producing polyketides, a large class of secondary metabolites — in other words, natural products. Two papers from Georgios Skiniotis and colleagues use cryo-electron microscopy to probe the structure of an intact module of a full-length multienzyme PKS module involved in pikromycin biosynthesis in the bacterium Streptomyces venezuelae in different functional states. The structures reveal how the ketosynthase, acyltransferase, ketoreductase and acyl carrier protein (ACP) domains interact during the catalytic cycle. In each state the ACP is differentially positioned to facilitate intermediate transfer for the next catalytic step and for transfer to the next module. The polyketide synthase (PKS) mega-enzyme assembly line uses a modular architecture to synthesize diverse and bioactive natural products that often constitute the core structures or complete chemical entities for many clinically approved therapeutic agents1. The architecture of a full-length PKS module from the pikromycin pathway of Streptomyces venezuelae creates a reaction chamber for the intramodule acyl carrier protein (ACP) domain that carries building blocks and intermediates between acyltransferase, ketosynthase and ketoreductase active sites (see accompanying paper2). Here we determine electron cryo-microscopy structures of a full-length pikromycin PKS module in three key biochemical states of its catalytic cycle. Each biochemical state was confirmed by bottom-up liquid chromatography/Fourier transform ion cyclotron resonance mass spectrometry. The ACP domain is differentially and precisely positioned after polyketide chain substrate loading on the active site of the ketosynthase, after extension to the β-keto intermediate, and after β-hydroxy product generation. The structures reveal the ACP dynamics for sequential interactions with catalytic domains within the reaction chamber, and for transferring the elongated and processed polyketide substrate to the next module in the PKS pathway. During the enzymatic cycle the ketoreductase domain undergoes dramatic conformational rearrangements that enable optimal positioning for reductive processing of the ACP-bound polyketide chain elongation intermediate. These findings have crucial implications for the design of functional PKS modules, and for the engineering of pathways to generate pharmacologically relevant molecules.

A palladium-catalysed C–H bond activation process is reported that proceeds through a four-membered-ring cyclopalladation pathway; it allows a methyl group that is adjacent to an unprotected secondary amine to be transformed into a synthetically versatile nitrogen heterocycle. C–H activation is a pivotal area of chemical synthesis and the discovery of new activation modes is crucial to its continued advancement. In this paper the authors report a palladium-catalysed C–H bond activation mode that proceeds through a four-membered ring cyclopalladation pathway rather than the kinetically favoured five-membered ring intermediates. The chemistry involved leads to the selective transformation of a methyl group that is adjacent to an unprotected secondary amine into a synthetically versatile nitrogen heterocycle. The scope of this previously unknown bond disconnection is highlighted through the development of C–H amination and carbonylation processes, leading to the synthesis of aziridines and β-lactams, respectively. The development of new chemical transformations based on catalytic functionalization of unactivated C−H bonds has the potential to simplify the synthesis of complex molecules dramatically. Transition metal catalysis has emerged as a powerful tool with which to convert these unreactive bonds into carbon−carbon and carbon−heteroatom bonds1,2,3,4,5,6, but the selective transformation of aliphatic C−H bonds is still a challenge. The most successful approaches involve a ‘directing group’, which positions the metal catalyst near a particular C−H bond, so that the C−H functionalization step occurs via cyclometallation7. Most directed aliphatic C−H activation processes proceed through a five-membered-ring cyclometallated intermediate8,9,10. Considering the number of new reactions that have arisen from such intermediates, it seems likely that identification of distinct cyclometallation pathways would lead to the development of other useful chemical transformations11. Here we report a palladium-catalysed C−H bond activation mode that proceeds through a four-membered-ring cyclopalladation pathway. The chemistry described here leads to the selective transformation of a methyl group that is adjacent to an unprotected secondary amine into a synthetically versatile nitrogen heterocycle. The scope of this previously unknown bond disconnection is highlighted through the development of C−H amination and carbonylation processes, leading to the synthesis of aziridines and β-lactams (respectively), and is suggestive of a generic C−H functionalization platform that could simplify the synthesis of aliphatic secondary amines, a class of small molecules that are particularly important features of many pharmaceutical agents.

Sulphide oxidation coupled to carbonate dissolution can provide a transient source of carbon dioxide to Earth’s atmosphere and so balance the Cenozoic increase in carbon dioxide consumption by silicate weathering, reconciling this increase with the need for mass balance in the long-term carbon cycle. It is thought that mountain uplift stimulated carbon dioxide consumption by silicate mineral weathering during the Cenozoic era, roughly spanning the past 66 million years, but there are no signs of a corresponding increase in volcanic carbon dioxide emissions that could balance the carbon budget. This paper suggests that some of the missing carbon dioxide could have been produced by coupled sulphide oxidation and carbonate dissolution, a process that may also have been accelerated in response to uplift. This hypothesis is consistent with isotopic records and may help explain the interactions between the long-term carbon cycle, tectonics, and Earth's climate. The observed stability of Earth’s climate over millions of years is thought to depend on the rate of carbon dioxide (CO2) release from the solid Earth being balanced by the rate of CO2 consumption by silicate weathering1. During the Cenozoic era, spanning approximately the past 66 million years, the concurrent increases in the marine isotopic ratios of strontium, osmium and lithium2,3,4 suggest that extensive uplift of mountain ranges may have stimulated CO2 consumption by silicate weathering5, but reconstructions of sea-floor spreading6 do not indicate a corresponding increase in CO2 inputs from volcanic degassing. The resulting imbalance would have depleted the atmosphere of all CO2 within a few million years7. As a result, reconciling Cenozoic isotopic records with the need for mass balance in the long-term carbon cycle has been a major and unresolved challenge in geochemistry and Earth history. Here we show that enhanced sulphide oxidation coupled to carbonate dissolution can provide a transient source of CO2 to Earth’s atmosphere that is relevant over geological timescales. Like drawdown by means of silicate weathering, this source is probably enhanced by tectonic uplift, and so may have contributed to the relative stability of the partial pressure of atmospheric CO2 during the Cenozoic. A variety of other hypotheses8,9,10 have been put forward to explain the ‘Cenozoic isotope-weathering paradox’, and the evolution of the carbon cycle probably depended on multiple processes. However, an important role for sulphide oxidation coupled to carbonate dissolution is consistent with records of radiogenic isotopes2,3, atmospheric CO2 partial pressure11,12 and the evolution of the Cenozoic sulphur cycle, and could be accounted for by geologically reasonable changes in the global dioxygen cycle, suggesting that this CO2 source should be considered a potentially important but as yet generally unrecognized component of the long-term carbon cycle.

Structures of budding yeast RNA polymerase I in a catalytically active conformation are presented and confirmed by visualizing processive transcription along ribosomal DNA genes; they support a general model for transcription elongation in which contracted and expanded polymerase conformations are associated with active and inactive states, respectively. This paper presents the long-sought structure of active, transcribing RNA polymerase I (Pol I), the enzyme that catalyses the first step in ribosome biogenesis—the transcription of ribosomal DNA—and regulates eukaryotic cell growth. Here, Achilleas Frangakis and colleagues have determined the structures of budding yeast Pol I in a catalytically active conformation using two different techniques: cryo-electron microscopy of single particles in vitro, and cryo-electron tomography of enzymes transcribing cellular ribosomal DNA under near-physiological conditions. The structures show that the active enzyme adopts a closed active centre conformation and support a general model for transcription elongation where contracted and expanded polymerase conformations are associated with active and inactive states, respectively. RNA polymerase I (Pol I) is a highly processive enzyme that transcribes ribosomal DNA (rDNA) and regulates growth of eukaryotic cells1,2,3,4. Crystal structures of free Pol I from the yeast Saccharomyces cerevisiae have revealed dimers of the enzyme stabilized by a ‘connector’ element and an expanded cleft containing the active centre in an inactive conformation5,6,7. The central bridge helix was unfolded and a Pol-I-specific ‘expander’ element occupied the DNA-template-binding site. The structure of Pol I in its active transcribing conformation has yet to be determined, whereas structures of Pol II and Pol III have been solved with bound DNA template and RNA transcript8,9,10. Here we report structures of active transcribing Pol I from yeast solved by two different cryo-electron microscopy approaches. A single-particle structure at 3.8 A resolution reveals a contracted active centre cleft with bound DNA and RNA, and a narrowed pore beneath the active site that no longer holds the RNA-cleavage-stimulating domain of subunit A12.2. A structure at 29 A resolution that was determined from cryo-electron tomograms of Pol I enzymes transcribing cellular rDNA confirms contraction of the cleft and reveals that incoming and exiting rDNA enclose an angle of around 150°. The structures suggest a model for the regulation of transcription elongation in which contracted and expanded polymerase conformations are associated with active and inactive states, respectively.

Saccharomyces cerevisiae bearing engineered alginate and mannitol catabolic pathways can ferment sugars from brown macroalgae to produce ethanol, potentially allowing the use of brown macroalgae as a viable feedstock for the production of biofuels and renewable chemicals. Brown macroalgae are seen as a viable feedstock for the production of biofuels, with the advantage that they can be farmed in coastal waters without using valuable arable land. However, the most abundant sugars in brown macroalgae are alginate, mannitol and glucan, and the full potential of this feedstock cannot be realized without extensive re-engineering of the alginate and mannitol catabolic pathways in Saccharomyces cerevisiae. In this paper the authors identify a 4-deoxy-L-erythro-5-hexoseulose uronate transporter in Asteromyces cruciatus brown algae and use it to develop a S. cerevisiae strain that can use the unique sugars in brown macroalgae for high-efficiency ethanol fermentation. With appropriate genetic modifications, this synthetic biology platform can be used to produce many other biofuels and renewable chemicals. The increasing demands placed on natural resources for fuel and food production require that we explore the use of efficient, sustainable feedstocks such as brown macroalgae. The full potential of brown macroalgae as feedstocks for commercial-scale fuel ethanol production, however, requires extensive re-engineering of the alginate and mannitol catabolic pathways1,2,3 in the standard industrial microbe Saccharomyces cerevisiae. Here we present the discovery of an alginate monomer (4-deoxy-l-erythro-5-hexoseulose uronate, or DEHU) transporter from the alginolytic eukaryote Asteromyces cruciatus4. The genomic integration and overexpression of the gene encoding this transporter, together with the necessary bacterial alginate and deregulated native mannitol catabolism genes, conferred the ability of an S. cerevisiae strain to efficiently metabolize DEHU and mannitol. When this platform was further adapted to grow on mannitol and DEHU under anaerobic conditions, it was capable of ethanol fermentation from mannitol and DEHU, achieving titres of 4.6% (v/v) (36.2 g l−1) and yields up to 83% of the maximum theoretical yield from consumed sugars. These results show that all major sugars in brown macroalgae can be used as feedstocks for biofuels and value-added renewable chemicals in a manner that is comparable to traditional arable-land-based feedstocks.

In comparison to primitive, chondritic meteorites, which are widely thought to be the building blocks of Earth, Earth and other differentiated planetary bodies are chemically fractionated, with Earth's crust and mantlethe 'silicate Earth'being strongly depleted in moderately volatile elements (such as lead, zinc, indium and the alkali metals). Two papers in this week's issue suggest that this difference in composition between chondritic meteorites and Earth could be a natural consequence of vapour loss from magma on the surface of growing planetesimals. Ashley Norris and Bernard Wood examined the melting processes that would have occurred during accretion on Earth and its precursor bodies and performed vaporization experiments under conditions of fixed temperature and oxygen partial pressure. They found that the pattern of volatile-element depletion in the silicate Earth is consistent with partial melting and vaporization rather than with simple accretion of a volatile-rich chondrite-like body. Remco Hin and co-authors show that differentiated planetary bodies have isotopically heavier magnesium compositions compared to chondritic meteorites, and conclude that this could be due to the isotopic fractionation between liquid and vapour, followed by vapour escape during accretionary growth of planetesimals.

The repressor of phage 434 binds to six operator sites on the phage chromosome. A comparison of the sequences of these 14-base-pair (bp) operator sites reveals a striking pattern: at five of the six sites, the symmetrically arrayed outer eight base pairs (four in each half-site) are identical and the remaining site differs at only one position (Fig. 1b). In contrast, the sequences of the inner four base pairs are highly variable. Crystallographic analysis of the repressor-operator complex shows that at each half-site, the 'recognition alpha-helix' of the repressor is positioned in the major groove such that it could contact the outermost five base pairs, but not the innermost two (Fig. 1a). We show in this paper that the sequence of the central base pairs of the operator (two in each half-site) have a significant role in determining operator affinity for repressor, despite the evidence presented here and in the accompanying paper that these base pairs are not contacted by repressor. We also show that these central base pairs influence operator affinity for Cro, a second gene regulatory protein encoded by phage 434. We discuss the likely structural basis for this evidently indirect, but sequence-dependent, effect of the central base pairs of the operator on its affinity for the two regulatory proteins.

A series of porous crystalline materials known as metal–organic materials are prepared, and a full sorption study shows that controlled pore size (rather than large surface area) coupled with appropriate chemistry lead to materials exhibiting fast and highly selective CO2 sorption. Metal organic frameworks are porous crystalline materials widely studied as potential gas separation and storage materials for clean energy applications. A general trend in this field has been the development of materials with the largest possible surface area with the aim of maximizing uptake of gases. In this paper the authors generate a series of metal organic frameworks and carry out sorption experiments that suggest that surface area may not be as important as was thought. Rather, pore size, coupled with appropriate chemistry, are the keys to fast CO2 uptake and strong CO2 sorption. Materials designed on these principles attain high selectivity for CO2 over nitrogen, oxygen, methane and hydrogen even in the presence of moisture. The energy costs associated with the separation and purification of industrial commodities, such as gases, fine chemicals and fresh water, currently represent around 15 per cent of global energy production, and the demand for such commodities is projected to triple by 2050 (ref. 1). The challenge of developing effective separation and purification technologies that have much smaller energy footprints is greater for carbon dioxide (CO2) than for other gases; in addition to its involvement in climate change, CO2 is an impurity in natural gas, biogas (natural gas produced from biomass), syngas (CO/H2, the main source of hydrogen in refineries) and many other gas streams. In the context of porous crystalline materials that can exploit both equilibrium and kinetic selectivity, size selectivity and targeted molecular recognition are attractive characteristics for CO2 separation and capture, as exemplified by zeolites 5A and 13X (ref. 2), as well as metal–organic materials (MOMs)3,4,5,6,7,8,9. Here we report that a crystal engineering7 or reticular chemistry5,9 strategy that controls pore functionality and size in a series of MOMs with coordinately saturated metal centres and periodically arrayed hexafluorosilicate (SiF62−) anions enables a ‘sweet spot’ of kinetics and thermodynamics that offers high volumetric uptake at low CO2 partial pressure (less than 0.15 bar). Most importantly, such MOMs offer an unprecedented CO2 sorption selectivity over N2, H2 and CH4, even in the presence of moisture. These MOMs are therefore relevant to CO2 separation in the context of post-combustion (flue gas, CO2/N2), pre-combustion (shifted synthesis gas stream, CO2/H2) and natural gas upgrading (natural gas clean-up, CO2/CH4).

Aerotaxy, an aerosol-based growth method, is used to produce gallium arsenide nanowires with a growth rate of about 1 micrometre per second, which is 20 to 1,000 times higher than previously reported for traditional nanowires and allows sensitive and reproducible control of the nanowires’ optical and electronic properties. Nanowires hold promise for a variety of applications in electronics, energy and biomedical technologies. However, a major hurdle is the large-scale production of high-quality nanowires. In this paper, Lars Samuelson and colleagues develop a low-cost aerosol-based synthesis of gallium arsenide (GaAs) nanowires with throughput substantially better than achieved by conventional methods. The method produces high-quality nanowires with tunable dimensions, good optical properties and spectral uniformity. Semiconductor nanowires are key building blocks for the next generation of light-emitting diodes1, solar cells2 and batteries3. To fabricate functional nanowire-based devices on an industrial scale requires an efficient methodology that enables the mass production of nanowires with perfect crystallinity, reproducible and controlled dimensions and material composition, and low cost. So far there have been no reports of reliable methods that can satisfy all of these requirements. Here we show how aerotaxy, an aerosol-based growth method4, can be used to grow nanowires continuously with controlled nanoscale dimensions, a high degree of crystallinity and at a remarkable growth rate. In our aerotaxy approach, catalytic size-selected Au aerosol particles induce nucleation and growth of GaAs nanowires with a growth rate of about 1 micrometre per second, which is 20 to 1,000 times higher than previously reported for traditional, substrate-based growth of nanowires made of group III–V materials5,6,7. We demonstrate that the method allows sensitive and reproducible control of the nanowire dimensions and shape—and, thus, controlled optical and electronic properties—through the variation of growth temperature, time and Au particle size. Photoluminescence measurements reveal that even as-grown nanowires have good optical properties and excellent spectral uniformity. Detailed transmission electron microscopy investigations show that our aerotaxy-grown nanowires form along one of the four equivalent 〈111〉B crystallographic directions in the zincblende unit cell, which is also the preferred growth direction for III–V nanowires seeded by Au particles on a single-crystal substrate. The reported continuous and potentially high-throughput method can be expected substantially to reduce the cost of producing high-quality nanowires and may enable the low-cost fabrication of nanowire-based devices on an industrial scale.

A simple and accessible method of probing the nature of bonding on the very surface of a material is reported, using transmission electron microscopy: the technologically important compound strontium titanate is examined as an example. This paper reports a simple and accessible method of probing the nature of bonding on the very surface of a material, using technologically important strontium titanate as an example. The authors generated a wedge-shaped sample, so that the thickness of the sample decreases as a transmission electron microscope tracks across it collecting spectroscopic information. In this way, the contribution of the signal from the bulk material decreases and that from the surface increases. The relative weights of the surface and bulk signals can be estimated from a series of spectral acquisitions, allowing the surface component of the data to be extracted. The determination of the atomic structure and the retrieval of information about reconstruction and bonding of metal oxide surfaces is challenging owing to the highly defective structure and insulating properties of these surfaces. Transmission electron microscopy (TEM) offers extremely high spatial resolution (less than one angstrom) and the ability to provide systematic information from both real and reciprocal space. However, very few TEM studies1,2,3 have been carried out on surfaces because the information from the bulk dominates the very weak signals originating from surfaces. Here we report an experimental approach to extract surface information effectively from a thickness series of electron energy-loss spectra containing different weights of surface signals, using a wedge-shaped sample. Using the (001) surface of the technologically important compound strontium titanate, SrTiO3 (refs 4, 5, 6), as a model system for validation, our method shows that surface spectra are sensitive to the atomic reconstruction and indicate bonding and crystal-field changes surrounding the surface Ti cations. Very good agreement can be achieved between the experimental surface spectra and crystal-field multiplet calculations based on the proposed atomic surface structure optimized by density functional calculations3. The distorted TiO6−x units indicated by the proposed model can be viewed directly in our high-resolution scanning TEM images. We suggest that this approach be used as a general method to extract valuable spectroscopic information from surface atoms in parallel with high-resolution images in TEM.