Search

The social amoebae are exceptional in their ability to alternate between unicellular and multicellular forms. Here we describe the genome of the best-studied member of this group, Dictyostelium discoideum. The gene-dense chromosomes of this organism encode approximately 12,500 predicted proteins, a high proportion of which have long, repetitive amino acid tracts. There are many genes for polyketide synthases and ABC transporters, suggesting an extensive secondary metabolism for producing and exporting small molecules. The genome is rich in complex repeats, one class of which is clustered and may serve as centromeres. Partial copies of the extrachromosomal ribosomal DNA (rDNA) element are found at the ends of each chromosome, suggesting a novel telomere structure and the use of a common mechanism to maintain both the rDNA and chromosomal termini. A proteome-based phylogeny shows that the amoebozoa diverged from the animal-fungal lineage after the plant-animal split, but Dictyostelium seems to have retained more of the diversity of the ancestral genome than have plants, animals or fungi.

An investigation into the possible causes of childhood cancer has been carried out throughout England, Scotland and Wales over the period 1991-1998. All children known to be suffering from one or other type of the disease over periods of 4-5 years have been included, and control children matched for sex, age and area of residence have been selected at random from population registers. Information about both groups of children (with and without cancer) has been obtained from parental questionnaires, general practitioners' and hospital records, and from measurement of the extent of exposure to radon gas, terrestrial gamma radiation, and electric and magnetic fields. Samples of blood have also been obtained from the affected children and their parents and stored. Altogether 3,838 children with cancer, including 1,736 with leukaemia, and 7,629 unaffected children have been studied. Detailed accounts are given of the nature of the information obtained in sections describing the general methodology of the study, the measurement of exposure to ionizing and non-ionizing radiation, the classification of solid tumours and leukaemias, and the biological material available for genetic analysis.

An investigation into the possible causes of childhood cancer has been carried out throughout England, Scotland and Wales over the period 1991–1998. All children known to be suffering from one or other type of the disease over periods of 4–5 years have been included, and control children matched for sex, age and area of residence have been selected at random from population registers. Information about both groups of children (with and without cancer) has been obtained from parental questionnaires, general practitioners’ and hospital records, and from measurement of the extent of exposure to radon gas, terrestrial gamma radiation, and electric and magnetic fields. Samples of blood have also been obtained from the affected children and their parents and stored. Altogether 3838 children with cancer, including 1736 with leukaemia, and 7629 unaffected children have been studied. Detailed accounts are given of the nature of the information obtained in sections describing the general methodology of the study, the measurement of exposure to ionizing and non-ionizing radiation, the classification of solid tumours and leukaemias, and the biological material available for genetic analysis.

A computer method is described for the automatic classification of triterpanes and steranes into gross structural type from their mass spectral characteristics. The method has been applied to the spectra obtained by gas-chromatographic/mass-spectroscopic analysis of two mixtures of standards and of hydrocarbon fractions isolated from Green River and Messel oil shales. Almost all of the steranes and triterpanes identified previously in both shales were classified, in addition to a number of new components. The results indicate that classification of such alkanes is possible with a laboratory computer system. The method has application to diagenesis and maturation studies as well as to oil/oil and oil/source rock correlations in which rapid screening of large numbers of samples is required.

The distributions of the major ring C monoaromatic and triaromatic steroid hydrocarbons of a number of oils, biodegraded under natural conditions to differing extents, have been examined by combined gas chromatography-mass spectrometry. Three major effects are observed: (i) loss of low molecular weight triaromatics, possibly from preferential water solubility; (ii) preferential degradation of mono- and triaromatics with the original 20R biological configuration; (iii) resistance of low molecular monoaromatics to biodegradation in comparison with their high molecular weight counterparts. In the laboratory, aerobic biodegradation of aromatic steroid hydrocarbons has also been demonstrated. Some of the effects are similar to those observed in the field studies.

Detailed results are presented for the distributions of triterpenoid alkanes, acids and alcohols of the hopane family in ten samples of peat from three environments, and of contributing organisms in the case of Lyne of Skene, Scotland. Extended hopanoids of the beta alpha and alpha beta configuration appear at the very earliest stage of diagenesis. Such environments appear to be characterised by particular distributions of C32 hopanoid acids and alcohols, and a high preponderance of the C31 alpha beta hopane.

Results are presented from an organic geochemical investigation of a suite of rock samples taken from the Upper Kimmeridge Clay near Kimmeridge, Dorset. All samples contain immature organic matter of marine origin, although one horizon, the Whitestone Band, contains an additional secondary input of partially biodegraded mature hydrocarbons, due to an oil seepage of unknown origin. With the exception of increased relative abundances of 4-methylsteroidal hydrocarbons in the more organic-rich samples, the immature molecular distributions are very similar, suggesting a consistent source of organic matter. The results are in agreement with the palaeoenvironmental model proposed by Tyson et al. (1979) for the deposition of the Kimmeridge Clay, where the different lithologies are controlled by a fluctuating oxic/anoxic boundary, with only the organic-poor mudstones being deposited in relatively oxygenated waters.

Computerized gas chromatography - mass spectrometry (g.c.-m.s.) is now an essential tool in the analysis of the complex mixtures of lipids (geolipids) encountered in aquatic sediments, both Recent (less than 1 Ma (10 6 years) old) and ancient. Most geolipid studies have been performed in the e.i. mode at low resolution but the techniques now being applied include c.i. and h.r.m.s. The large quantities of data acquired from capillary g.c.-m.s. runs necessitate fast data acquisition and data processing, including the capability for the automatic selection and refinement of key spectra. Even so, the chemist is faced with the identification and/or recognition of at least several hundred good quality spectra from a single run. Fast routine search procedures are useful here, especially for known compounds, while classification routines based on established rules for manual interpretation can be of assistance even with novel compounds. Examples from recent studies (at Bristol) of contemporary, Recent and ancient sediments, are presented. Geolipids show abundance patterns of homologous series which, while related to those of known organisms, display many novel features, including extensive carbon number ranges and stereospecific distributions. Additionally, certain carbon skeleton types, commonly thought to be rare as natural products, are major components of geolipid fractions, presumably reflecting the composite inputs and early microbial diagenetic activity.

Solid state 13C nuclear magnetic resonance (NMR) spectroscopy has been applied to the examination of source rocks, demineralised kerogens and oils. In addition, a range of source rocks and kerogens have been examined by pyrolysis-mass spectrometry. Using solid state NMR techniques a measure of the aromaticity of a sample can be obtained. This can be used to “type” immature kerogens or, in the case of a given source rock (a Cretaceous limestone from the Middle East), it can be used to monitor changes in the maturity of the formation across the basin in which it occurs. A steady increase in aromaticity is observed up to the oil generation threshold, followed by a faster increase in aromaticity at greater depths. This faster increase in aromaticity is due to the onset of generation and migration of hydrocarbons. Oils sourced by this formation have been examined in solution by 13C NMR and a variation in aromaticity complementing that seen in the sediments is observed. Preliminary work using pyrolysis-mass spectrometry has shown that it may be of value in the rapid typing of kerogens. It has the advantage over NMR of rapid processing of samples and possible use without prior demineralisation of samples.

Almost an of the solvent-extractable sterols and their nuclearsaturated analogues in a sample of Walvis Bay surface sediment have been analysed by capillary GLC and GC-MS, and by coinjection with a variety of standards. The presence in sediments of 22- trans -24-nor-5α-cholest-22-en-3β-ol, 24-methylene-5α-cholestan-3β-ol, and components tentatively assigned as 23,24-dimethylcholesta-5,22-dien-3β-ol and 23,24-dimethyl-5α-cholest-22-en-3β-ol has been demonstrated for the first time. A novel sterol and its saturated analogue have also been found. The sterol distribution cannot be related solely to the reported major input of phytoplankton; the presence of 22,23-methylene-23,24-dimethylcholest-5-en-3β-ol and its saturated analogue indicates a coelenterate contribution. The analysis emphasises the necessity of glass capillary columns and coinjection of standards.

The widespread occurrence of extended hopanoids in sediments and petroleums illustrates the importance of bacterial lipid contributions to geological materials1. In archaebacteria, however, hopanoids are absent1; their role as structural components of biomembranes is fulfilled by acyclic isoprenoids1–5. Recent studies2–6 of the lipid constituents of archaebacteria have greatly extended the range of acyclic isoprenoid skeletons known in organisms (Fig. 1). In particular, isoprenoids with head-to-head linkages have been identified3–5, and such compounds (for example, 3,7,11,15,18,22,26,30-octamethyldotriacontane, I) have been recognized in petroleums7,8 and as degradation products of Messel shale kerogen9–11. Here we report the first recognition of 2,6,10,15,19-pentamethyleicosane (II), a known component of methanogens5,6, in marine sediments of Recent to Cretaceous age (Table 1) and suggest that it and certain other acyclic isoprenoids may be used as biological markers for methanogens.

Sedimentary C32 hopanoic acid, one of the most abundant in nature and of probable bacterial origin, has been isolated for the first time as a single component and characterized by H-1 NMR. The 17 alpha H, 21 beta H configuration of the C31 alkane has been similarly confirmed.