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During acclimation to high temperature, photosynthetic organisms enhance the thermal stability of their photosynthetic activity (1). This phenomenon has been observed in a number of species of plants (1) and cyanobacteria (2,3), whereas the physiological importance and the underlying mechanism of the acclimative response are unclear. The oxygen-evolving machinery of the photosystem II (PS II) complex is known to be the most susceptible to high temperature among various components of the photosynthetic machinery (1,4). We reported previously that in the PS II complex only the reaction of the evolution of oxygen was stabilized during acclimation to high temperature in the cyanobacterium Synechococcus sp. PCC 7002 (3). Therefore, it is likely that the stabilization of the oxygen-evolving machinery from heat-induced inactivation would be the mechanism that underlies the photosynthetic acclimation. We showed previously that thylakoid membranes isolated from cells of Synechococcus sp. PCC 7002, which had been grown at high temperatures, exhibited a greater thermal stability in the oxygen-evolving activity than those from cells grown at low temperatures (3). This finding suggested that factors for the thermal stability of the oxygen-evolving machinery were associated with thylakoid membranes. Biochemical investigations of the thylakoid membranes identified two proteins, cytochrome C550 and PsbU, as such factors (5,6). Cytochrome C550 and PsbU are the extrinsic proteins of the PS II complex that have been found in several species of cyanobacteria (7,8) and red algae (9), but not in higher plants (10). In the present study we inactivated the psbU gene in Synechococcus sp. PCC 7002 by targeted mutagenesis in order to examine the thermoprotective role of PsbU in vivo particularly, in terms of acclimation to high temperature.

Cyanobacteria exhibit considerable tolerance to salt stress and are useful for studies of acclimation and tolerance to such stress. In a previous study we isolated a desA−/desD− mutant strain of Synechocystis sp. PCC 6803 in which the desA and desD genes for the Δl2 and Δ6 desaturases, respectively, had been inactivated by targeted mutagenesis (1). The desA−/desD− cells contain monounsaturated but not polyunsaturated fatty acids, whereas wild-type cells contain polyunsaturated fatty acids such as di-, tri-, and tetraunsaturated fatty acids (2,3). In the present study, we investigated the contribution of the unsaturation of fatty acids in membrane lipids to tolerance to salt stress by comparing desA − /desD − cells to wild-type cells of Synechocystis sp. PCC 6803. We demonstrated that the unsaturation of fatty acids is associated with the ability of the photosynthetic machinery to tolerate salt stress.

The role of membrane lipid unsaturation in the restoration of photosystem II (PSII) function and in the synthesis of the D1 protein at different temperatures after photoinhibition was studied in wild-type cells and a mutant of Synechocystis sp. PCC 6803 with genetically inactivated desaturase genes. We show that posttranslational carboxyl-terminal processing of the precursor form of the D1 protein is an extremely sensitive reaction in the PSII repair cycle and is readily affected by low temperature. Furthermore, the threshold temperature at which perturbations in D1-protein processing start to emerge is specifically dependent on the extent of thylakoid membrane lipid unsaturation, as indicated by comparison of wild-type cells with the mutant defective in desaturation of 18:1 fatty acids of thylakoid membranes. When the temperature was decreased from 33[deg]C (growth temperature) to 18[deg]C, the inability of the fatty acid mutant to recover from photoinhibition was accompanied by a failure to process the newly synthesized D1 protein, which accumulated in considerable amounts as an unprocessed precursor D1 protein. Precursor D1 integrated into PSII monomer and dimer complexes even at low temperatures, but no activation of oxygen evolution occurred in these complexes in mutant cells defective in fatty acid unsaturation.

Glycinebetaine is one of the compatible solutes that accumulate in the chloroplasts of contain halotolerant plants when these plants are exposed to salt or cold stress. The codA gene for choline oxidase, the enzyme that converts choline into glycinebetaine, has previously been cloned from a soil bacterium, Arthrobacter globiformis. Transformation of Arabidopsis thaliana with the cloned codA gene under the control of the 35S promoter of cauliflower mosaic virus enabled the plant to accumulate glycinebetaine and enhanced its tolerance to salt and cold stress. At 300 mM NaCl, considerable proportions of seeds of transformed plants germinated well, whereas seeds of wild-type plants failed to germinate. At 100 mM NaCl, transformed plants grew well whereas wild-type plants did not do so. The transformed plants tolerated 200 mM NaCl, which was lethal to wild-type plants. After plants had been incubated with 400 mM NaCl for two days, the photosystem II activity of wild-type plants had almost completely disappeared, whereas that of transformed plants remained at more than 50% of the original level. When exposed to a low temperature in the light, leaves of wild-type plants exhibited symptoms of chlorosis, whereas those of transformed plants did not. These observations demonstrate that the genetic modification of Arabidopsis thaliana that allowed it to accumulate glycinebetaine enhanced its ability to tolerate salt and cold stress.

The cyanobacterium Synechococcus sp. strain PCC 7942 was transformed with the codA gene for choline oxidase from Arthrobacter globiformis under the control of a constitutive promoter. This transformation allowed the cyanobacterial cells to accumulate glycine betaine at 60 to 80 mM in the cytoplasm. The transformed cells could grow at 20 degrees C, the temperature at which the growth of control cells was markedly suppressed. Photosynthesis of the transformed cells at 20 degrees C was more tolerant to light than that of the control cells. This was caused by the enhanced ability of the photosynthetic machinery in the transformed cells to recover from low-temperature photoinhibition. In darkness, photosynthesis of the transformed cells was more tolerant to low temperature such as 0 to 10 degrees C than that of the control cells. In parallel with the improvement in the ability of the transformed cells to tolerate low temperature, the lipid phase transition of plasma membranes from the liquid-crystalline state to the gel state shifted toward lower temperatures, although the level of unsaturation of the membrane lipids was unaffected by the transformation. These findings suggest that glycine betaine enhances the tolerance of photosynthesis to low temperature.

According to current schemes of the glycerolipid synthesis, fatty acids are synthesized and then desaturated in plastids (1). Since the activity of UDP-galactose:1,2-diacylglycerol galactosyltransferase, which is involved in the biosynthesis of galactolipids, was first found in envelope membranes isolated from chloroplasts (2), it has been assumed that the sites of synthesis of glycerolipids and of the desaturation of fatty acids in plastids are confined to the envelope membranes and are not present in the thylakoid membranes (3–5). However, several lines of evidence suggest that the thylakoid membranes might also be a site of glycerolipid synthesis and desaturation (6–8).

Acyl-lipid desaturases introduce double bonds (unsaturated bonds) at specifically defined positions in fatty acids that are esterified to the glycerol backbone of membrane glycerolipids. The desA, desB and desD genes of Synechocystis sp. PCC 6803 encode acyl-lipid desaturases that introduce double bonds at the delta12, omega3 and delta6 positions of C18 fatty acids respectively. The mutation of each of these genes by insertion of an antibiotic resistance gene cartridge completely eliminated the corresponding desaturation reaction. This system allowed us to manipulate the number of unsaturated bonds in membrane glycerolipids in this organism in a step-wise manner. Comparisons of the variously mutated cells revealed that the replacement of all polyunsaturated fatty acids by a monounsaturated fatty acid suppressed growth of the cells at low temperature and, moreover, it decreased the tolerance of the cells to photoinhibition of photosynthesis at low temperature by suppressing recovery of the photosystem II protein complex from photoinhibitory damage. However, the replacement of tri- and tetraunsaturated fatty acids by a diunsaturated fatty acid did not have such effects. These findings indicate that polyunsaturated fatty acids are important in protecting the photosynthetic machinery from photoinhibition at low temperatures.

The Fad12 mutant of Synechocystis sp. PCC 6803 has a defect in the desA gene for delta 12 acyl-lipid desaturase. We identified a change in the nucleotide sequence of the structural gene for the desaturase, in which a leucine codon has been converted to a stop codon. Western blot analysis revealed that the delta 12 acyl-lipid desaturase was localized in both plasma membranes and thylakoid membranes of wild-type cells but was absent from both types of membrane in Fad12 cells. These findings suggest that the desaturation of fatty acids takes place in both types of membrane in Synechocystis sp. PCC 6803. The mutation in the delta 12 desaturase did not affect the lipid composition of thylakoid and plasma membranes, but it changed the fatty acid composition of lipids in similar ways in both types of membrane.

Most cyanobacteria and plants contain high levels of polyunsaturated fatty acids of membrane lipids. We isolated the desA, desB and desC genes of Synechocystis sp. PCC 6803, which encode the Δ12, Δ15 (ω3) and Δ9 desaturases, respectively, of the acyl-lipid type. We disrupted the desA gene and the desD gene (for Δ6 desaturase) in Synechocystis sp. PCC 6803 by insertion of antibiotic resistance gene cartridges. These mutations greatly modified the extent of unsaturation of the fatty acids of membrane lipids. These changes in the unsaturation of membrane lipids altered the tolerance of cyanobacterial cells to low temperature. In higher plants, we isolated a cDNA for acyl-ACP: glycerol-3-phosphate acyltransferase from squash and introduced it into tobacco plants. The transformation modified the level of unsaturation of chloroplastic phosphatidylglycerol, and altered the tolerance of tobacco plants to low temperature. Glycinebetaine, a compatible solute, is synthesized in some halophilic plants. We cloned, from the soil bacterium Arthrobacter globiformis, the codA gene for choline oxidase which converts choline into glycinebetaine. introduced this gene into Synechococcus sp. PCC 7942, the resultant transformants accumulated glycinebetaine to some extent and became tolerant to salinity stresses.

Natural osmoregulatory substances (osmolytes) allow a wide variety of organisms to adjust to environments with high salt and/or low water content. In addition to their role in osmoregulation, some osmolytes protect proteins from denaturation and deactivation by, for example, elevated temperature and chaotropic compounds. A ubiquitous protein-stabilizing osmolyte is glycine betaine (N-trimethyl glycine). Its presence has been reported in bacteria, in particular cyanobacteria, in animals and in plants from higher plants to algae. In the present review we describe the experimental evidence related to the ability of glycine betaine to enhance and stabilize the oxygen-evolving activity of the Photosystem II protein complexes of higher plants and cyanobacteria. The osmolyte protects the Photosystem II complex against dissociation of the regulatory extrinsic proteins (the 18 kD, 23 kD and 33 kD proteins of higher plants and the 9 kD protein of cyanobacteria) from the intrinsic components of the Photosystem II complex, and it also stabilizes the coordination of the Mn cluster to the protein cleft. By contrast, glycine betaine has no stabilizing effect on partial photosynthetic processes that do not involve the oxygen-evolving site of the Photosystem II complex. It is suggested that glycine betaine might act, in part, as a solute that is excluded from charged surface domains of proteins and also as a contact solute at hydrophobic surface domains.

Desaturases introduce double bonds into fatty acids. They are important in the regulation of the degree of unsaturation of membrane glycerolipids and, thus, in the ability of certain organisms to tolerate low temperatures [1,2,3,4,5]. There are three types of desaturase, as follows. (1) Acyl-CoA desaturases introduce double bonds into fatty acids bound to coenzyme A, and these enzymes are bound to the endoplasmic reticulum in animal, yeast and fungal cells. (2) Acyl-ACP desaturases introduce double bonds into fatty acids that are bound to acyl-carrier protein and they are present in the stroma in plant chloroplasts. (3) Acyl-lipid desaturases introduce double bonds into fatty acids that have been esterified to glycerolipids, and they are bound to the membranes of plant and cyanobacterial cells. This last type of desaturase is the most efficient regulator of the extent of the unsaturation of membrane lipids in response to changes in temperature.

Biological organisms are capable of perceiving environmental changes caused by a variety of agents. But whereas sensors of light (rhodopsins in animals or phytochromes in plants) are well-characterized, no knowledge is available concerning the thermal sensors. The only clue hitherto known about the thermal perception is that temperature decrease would result in a reduction of molecular motion in membranes, which is attained mainly via specific desaturation of fatty acids of membrane lipids [1]. It is assumed, that the response of systems to temperature stress via physiological, morphological and biochemical adjustments aimed at offset or compensate for the temperature-induced disturbances. Amongst the many compensatory responses, it seems that the major adjustment is the reorganization of the physical state of the cellular membranes, a phenomenon called “homeoviscous adaptation”.

Microorganisms respond to hyperosmotic stress via changes in the levels of expression of large numbers of genes. Such responses are essential for acclimation to a new osmotic environment. To identify factors involved in the perception and transduction of signals caused by hyperosmotic stress, we examined the response of Synechocystis sp. PCC 6803, which has proven to be a particularly useful microorganism in similar analyses. We screened knockout libraries of histidine kinases (Hiks) and response regulators (Rres) in Synechocystis by DNA microarray and slot-blot hybridization analyses, and we identified several two-component systems, which we designated Hik-Rre systems, namely, Hik33-Rre31, Hik34-Rre1, and Hik10-Rre3, as well as Hik16-Hik41-Rre17, as the transducers of hyperosmotic stress. We also identified Hik2-Rre1 as a putative additional two-component system. Each individual two-component system regulated the transcription of a specific group of genes that were responsive to hyperosmotic stress.

In cyanobacteria, the biosynthesis of unsaturated fatty acids is initiated by delta 9 acyl-lipid desaturase which introduces the first double bond at the delta 9 position of a saturated fatty acid that has been esterified to a glycerolipid. We have cloned genes, designated desC, for delta 9 acyl-lipid desaturases from two cyanobacteria, namely Anabaena variabilis and Synechocystis sp. PCC 6803. These desaturases, when expressed in Escherichia coli, desaturated stearic acid to yield oleic acid at the C-1 positions of phosphatidylethanolamine and phosphatidylglycerol, but did not desaturate palmitic acid, palmitoleic acid, and cis-vaccenic acid. These results indicate that the delta 9 acyl-lipid desaturases are specific to stearic acid esterified at the C-1 position of a glycerolipid and are nonspecific with respect to the polar head group of the glycerolipid. The deduced amino acid sequences of the delta 9 acyl-lipid desaturases are similar in part to those of stearoyl-CoA desaturases of the rat, the mouse, and Saccharomyces cerevisiae, but not to those of acyl-(acyl-carrier-protein) desaturases of higher plants.

We investigated the factors responsible for the heat stability of photosynthetic oxygen evolution by examining thylakoid membranes from the cyanobacterium Synechococcus sp. PCC 7002. We found that treatment of the thylakoid membranes with 0.1% Triton X-100 resulted in a remarkable decrease in the heat stability of oxygen evolution, and that the heat stability could be restored by reconstituting the membranes with the components that had been extracted by Triton X-100. The protein responsible for the restoration of heat stability was purified from the Triton X-100 extract by two successive steps of chromatography. The purified protein had a molecular mass of 16 kD and exhibited the spectrophotometric properties of a c-type Cyt with a low redox potential. The dithionite-minus-ascorbate difference spectrum revealed an alpha band maximum at 551 nm. We were able to clone and sequence the gene encoding this Cyt from Synechococcus sp. PCC 7002, based on the partial amino-terminal amino acid sequence. The deduced amino acid sequence revealed a gene product consisting of a 34-residue transit peptide and a mature protein of 136 residues. The mature protein is homologous to Cyt c550, a Cyt with a low redox potential. Thus, our results indicate that Cyt c550 greatly affects the heat stability of oxygen evolution.

Cyanobacterial genes for enzymes that desaturate fatty acids at the delta 12 position, designated desA, were isolated from Synechocystis PCC6714, Synechococcus PCC7002 and Anabaena variabilis by cross-hybridization with a DNA probe derived from the desA gene of Synechocystis PCC6803. The genes of Synechocystis PCC6714, Synechococcus PCC7002 and A. variabilis encode proteins of 349, 347 and 350 amino acid residues, respectively. The transformation of Synechococcus PCC7942 with the desA genes from Synechocystis PCC6714, Synechococcus PCC7002 and A. variabilis was associated with the ability to introduce a second double bond at the delta 12 position of fatty acids. The amino acid sequence of the products of the desA genes revealed the presence of four conserved domains. Since one of the conserved domains was also found in the amino acid sequences of omega 3 desaturases of Brassica napus and mung bean, this domain may play an essential role in the introduction of a double bond into fatty acids bound to membrane lipids.

We conducted the following research in order to understand molecular mechanism of tolerance and adaptation in higher plants and microbiral plants to high and low temperature. (1) Glycerol-3-phosphate acyltransferase from Arabidopsis and squash was introduced into tobacco, resulting in a modified chilling tolerance in the leaves of the transgenic tobacco. (2) Cells of a cyanobacterium, Synechococcus PCC7002, were grown under high temperature, and aquired thermotolerance in the photosynthetic oxygen-evolving complex. The factors responsible for the enhanced thermal stability appeared to be associated with the thylakoid membrane. (3) Two groEL-homologous genes were cloned from Synechococcus PCC7002 and were then sequenced. Heat shock treatment markedly increased the mRNAs of both groEL-homologous genes. One gene was accompanied by an upstream groES gene, while the other was not.

One of the well-characterized phenomena associated with the acclimation of organisms to changes in ambient temperature is the regulation of the molecular motion or "fluidity" of membrane lipids via changes in the extent of unsaturation of the fatty acids of membrane lipids. The enzymes responsible for this process when the temperature is decreased are the desaturases, the activities of which are enhanced at low temperature. To examine whether the change in the fluidity of membrane lipids is the first event that signals a change in temperature, we studied the effect of the Pd-catalyzed hydrogenation of membrane lipids on the expression of the desA gene, which is responsible for the desaturation of fatty acids of membrane lipids in the cyanobacterium Synechocystis PCC6803. The Pd-catalyzed hydrogenation of plasma membrane lipids stimulated the expression of the desA gene. We also found that, for unexplained reasons, the hydrogenation was much more specific to a minor phospholipid, phosphatidylglycerol, than to members of other lipid classes. These results suggest that the organism perceives a decrease in the fluidity of plasma membrane lipids when it is exposed to a decrease in temperature.

The rplI gene encoding the ribosomal protein L9 was found 4 kbp downstream from the desA gene, but on the opposite strand, in the genome of the cyanobacterium Synechocystis PCC6803. The deduced amino acid sequence is homologous to the sequences of the L9 proteins from Escherichia coli and chloroplasts of Arabidopsis and pea. The gene is present as a single copy in the chromosome and is transcribed as a mRNA of 0.64 kb. An open reading frame of unknown function (ORF291) was found in the upstream region of the rplI gene.

It has been stated that the fatty-acid unsaturation of membrane lipids is related to the tolerance (or sensitivity) of plants to high- and low-temperature stresses. This claim has not been supported directly in previous studies because the level of unsaturation has always been altered indirectly by changing the growth temperature. The change in growth temperature may be expected to alter not only the lipids, but also a number of other cellular components and enzymes. Therefore, to study the relationship between temperature tolerance and the lipid, it is necessary to establish an experimental system in which only the fatty-acid unsaturation of membrane lipids is altered, while the growth temperature remains constant. Such a system is now available through gene technology that modifies membrane lipids using genes for the desaturation of fatty acids. Recently, we have isolated genes termed desA and desB) for fatty-acid desaturation at the Δ12 and Δ15 positions from the cyanobacterium Synechocystis PCC6803. By mutagenesis and transformation with desaturase genes, we obtained strains of this cyanobacterium that had discretely different levels of fatty-acid unsaturation of their membrane lipids. The characteristic features of temperature tolerance of these strains suggest that the fatty-acid unsaturation of membrane lipids is related to the tolerance against low- temperature stress, but not to that against high-temperature stress. A second line of research has been the gene-technological modification of fatty-acid unsaturation of phosphatidylglycerol, one of the membrane lipids of higher- plant chloroplasts. In these studies, we cloned genes for acyl-ACP:glycerol- 3-phosphate acyltransferase from Arabidopsis thaliana and Cucurbita moschata (squash) and introduced them into Nicotiana tabacum (tobacco). Leaves of the resultant tobacco revealed distinctly different levels of unsaturation of the phosphatidylglycerol. By comparing the tolerance of the transformants against low-temperature stress, we have obtained evidence that low-temperature tolerance is enhanced by unsaturation of the membrane lipid.

Thylakoid membranes isolated from the cyanobacterium Synechocystis sp. strain PCC6803 were capable of desaturating the acyl groups in monogalactosyl diacylglycerol. This desaturation reaction required the reduced form of ferredoxin.

Acyl-(acyl-carrier-protein) hydrolase (EC 3.1.2.14) releases fatty acids from the end-product of fatty acid synthesis in plastids for the subsequent synthesis of glycerolipids in the cytoplasm. Isoelectric focusing of chloroplast stroma proteins from squash cotyledons suggested that there were at least three isomeric forms of acyl-(acyl-carrier-protein) hydrolase having pI values of 4.5, 5.3 and 7.8. The pI 4.5 and pI 5.3 forms showed maximum activity at pH 9.8 whereas the activity of the pI 7.8 form increased within the range 6.2 to 10.2 but no optimum was seen. The pI 4.5 form was purified 100,000-fold from squash cotyledons. The highly purified fraction contained two polypeptides, whose molecular masses were estimated to be 35 kDa and 33 kDa by SDS-PAGE. It is suggested that the 33 kDa polypeptide was a degradation product of the 35 kDa polypeptide. Oleoyl-(acyl-carrier protein) was the preferred substrate of this enzyme over palmitoyl- and stearoyl-(acyl-carrier protein), whereas lauroyl-(acyl-carrier protein) was nearly inactive. These results indicate the enzyme is specific for long-chain acyl-(acyl-carrier protein).

Vibrio sp. strain ABE-1 is a unique marine bacterium in terms of its ability to synthesize Δ9-trans-hexadecenoic acid and Δ7-cis-tetradecenoic acid (14:1(7c); Okuyama, H., Sasaki, S., Higashi, S. and Murata, N. (1990) J. Bacteriol. 172, 3515-3518). The present study, involving labeling with [1-14C]acetate, demonstrated that 14:1 is synthesized by the anaerobic pathway. When cells of this bacterium were grown in the presence of [1-14C]myristic acid (14:0), this compound was converted to palmitic (16:0) and hexadecenoic (16:1) acids but not to 14:1, under aerobic conditions. These results suggest that the incorporated 14:0 was elongated to 16:0 and then converted to 16:1 by the aerobic desaturation of 16:0. It appears that the anaerobic pathway and aerobic desaturation are both involved in the synthesis of unsaturated fatty acids during aerobic growth of Vibrio sp. strain ABE-1.

The photosynthetic oxygen-evolving activity of the photosystem 2 complex, prepared from spinach, was labile when the complex was exposed to high-salt conditions under which the extrinsic proteins were dissociated from the complex. Glycinebetaine prevented the dissociation of the 18-kDa and the 23-kDa extrinsic proteins from the photosystem 2 complex in the presence of 1 M NaCl. It also prevented the dissociation of the 33-kDa extrinsic protein from the complex in the presence of 1 M MgCl2 or 1 M CaCl2. The oxygen-evolving activity of the photosystem 2 complex was stabilized by glycinebetaine when the complex was subjected to treatment with NaCl and MgCl2.

The major phospholipid, phosphatidylethanolamine (PE), of Vibrio sp. strain ABE-1 contains a unique trans-unsaturated fatty acid, 9-trans-hexadecenoic acid (16:1(9t], at the sn-2 position of the glycerol moiety. The major molecular species of PE that contain 16:1(9t) at the sn-2 position have either 9-cis-hexadecenoic acid (16:1(9c] or hexadecanoic acid (16:0) at the sn-1 position. The transition temperatures of the liquid-crystal to the gel phase of 16:1(9c)/16:1(9t)-PE and 16:0/16:1(9t)-PE were -3 degrees C and 38 degrees C, respectively, temperatures that were 31 degrees C and 18 degrees C higher than the corresponding temperatures for 16:1(9c)/16:1(9c)-PE and 16:0/16:1(9c)-PE. The proportion of 16:1(9c)/16:1(9t)-PE and 16:0/16:1(9t)-PE increased significantly in cells grown at 20 degrees C over that in cells grown at 5 degrees C. When cells grown at 5 degrees C were incubated at 20 degrees C in the presence of cerulenin, an inhibitor of the synthesis de novo of fatty acids, the level of 16:1(9t) increased almost in parallel with a concomitant decrease in the level of 16:1(9c) at the sn-2 position. These results suggest that 16:1(9c) is converted to 16:1(9t) by the cis/trans isomerization of the double bond in the fatty acid. This conversion is discussed as a possible strategy for adaptation by bacteria to changes in temperature.

The phase transition in the lipid phase of aqueous dispersions of diacylglyceryltrimethylhomoserine (DGTS) was measured by fluorescence depolarization of parinaric acid and differential scanning calorimetry. In both techniques, the phase transition temperatures (Tm) of 1-palmitoyl-2-stearoyl DGTS and of 1,2-distearoyl DGTS were 53 and 59°C, respectively. Each of these Tm values was significantly higher than the Tm value of phosphatidylcholine with an identical combination of fatty acids. This suggests that the intermolecular interactions of DGTS molecules are slightly different from those of phosphatidylcholine molecules.

The lipid composition of Prochlorothrix hollandica, a photosynthetic prokaryote that contains chlorophylls a and b, was found to be similar to that of cyanobacteria and that of another member of the prochlorophyta, namely, Prochloron sp.

THE sensitivity (or tolerance) of plants to chilling determines their choice of natural habitat and also limits the worldwide production of crops. Although the molecular mechanism for chilling sensitivity has long been debated, no definitive conclusion has so far been reached about its nature. A probable hypothesis1,2, however, is that chilling injury is initiated by phase transition of lipids of cellular membranes, as demonstrated for cyanobacteria, which serve as a model system for the plant cells3,4. Because the phase transition temperature depends on the degree of unsaturation of fatty acids of the membrane lipids5, it is predicted that the chilling tolerance of plants can be altered by genetically manipulating fatty-acid desaturation by introducing double bonds into fatty acids of membrane lipids. Here we report the cloning of a gene for the plant-type desaturation (termed desA). The introduction of this gene from a chilling-resistant cyanobacterium, Synecho-cystis PCC6803, into a chilling-sensitive cyanobacterium, Anacystis nidulans, increases the tolerance of the recipient to low temperature.

Three extrinsic proteins, located on the luminal side of PSII, participate in oxygen evolution« Molecular masses for these proteins have been designated as 33, 23 and 18 kDa from SDS-PAGE (for a review see [1]). Partial degredation of the N-terminal portion of the 18- and 23-kDa proteins suggested that this region contains a domain for binding to the PSII complex [2,3]. In the present work we subjected the 33-kDa protein to limited proteolysis using chymotrypsin and Staphylococcus aureus (Strain V8) protease. These proteolytic enzymes eliminated 16 and 18 amino acid residues, respectively, from the N-terminal region of the protein. The remaining large fragments could not rebind to PSII.

Microorganisms living in highly saline aquatic environments face two survival tasks: to prevent H2O loss by osmosjs and to protect cytoplasmic structures against destruction by invading Cl−. Defensive strategies include accumulation by cellS of low molecular weight osmoregulatory substances (such as glycine betaine in Oscillatoria limnetica, ref. 1). Glycine betaine (hereafter betaine) has been shown to protect soluble enzymes against inactivation by high NaCl concentrations (2).

The thylakoid membrane is unique in high levels of characteristic glycolipids, MGDG, DGDG and SQDG and a low level of phospholipid, PG (1,2). The binding of specific lipids to the protein complexes from the thylakoid membranes has been studied; e.g., the association of SQDG and DGDG to the CF1CF0 complex (3), a phospholipid (probably PG) to cytochrome b6/f complex (4), and PG to the light-harvesting Ch1-protein complex (5).

The protective effect of 1 M glycinebetaine on thermal inactivation of photosynthetic oxygen evolution in isolated photosystem II membrane fragments from spinach is observed in CO2-free medium in both the presence and absence of added 2 mM bicarbonate. Conversely, the protective effect of 2 mM bicarbonate against thermoinactivation is seen in the absence as well as in the presence of 1 M glycinebetaine. The stabilizing effect of bicarbonate is also observed in thylakoid membranes from Synechococcus sp. PCC 7002 treated with 0.1% Triton X-100, and in unbroken spinach thylakoids. It is shown for the first time that bicarbonate protects the water-oxidizing complex against inactivation induced by pre-incubation of photosystem II membrane fragments (25°C) and thylakoids (40°C) at low pH (5.0–5.5) in non-bicarbonate-depleted medium. We conclude that the protective effects of glycinebetaine and bicarbonate are of a different nature; glycinebetaine acts as a non-specific, compatible, zwitterionic osmolyte while bicarbonate is considered an essential constituent of the water-oxidizing complex of photosystem II, important for its functioning and stabilization.