Enhancement of chilling tolerance of a cyanobacterium by genetic manipulation of fatty acid desaturation

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 hypothesis, 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 cells. Because the phase transition temperature depends on the degree of unsaturation of fatty acids of the membrane lipids, 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, Synechocystis PCC6803, into a chilling-sensitive cyanobacterium, Anacystis nidulans, increases the tolerance of the recipient to low temperature.

Unsaturation of the membrane lipids of chloroplasts stabilizes the photosynthetic machinery against low-temperature photoinhibition in transgenic tobacco plants

Using tobacco plants that had been transformed with the cDNA for glycerol-3-phosphate acyltransferase, we have demonstrated that chilling tolerance is affected by the levels of unsaturated membrane lipids. In the present study, we examined the effects of the transformation of tobacco plants with cDNA for glycerol-3-phosphate acyltransferase from squash on the unsaturation of fatty acids in thylakoid membrane lipids and the response of photosynthesis to various temperatures. Of the four major lipid classes isolated from the thylakoid membranes, phosphatidylglycerol showed the most conspicuous decrease in the level of unsaturation in the transformed plants. The isolated thylakoid membranes from wild-type and transgenic plants did not significantly differ from each other in terms of the sensitivity of photosystem II to high and low temperatures and also to photoinhibition. However, leaves of the transformed plants were more sensitive to photoinhibition than those of wild-type plants. Moreover, the recovery of photosynthesis from photoinhibition in leaves of wild-type plants was faster than that in leaves of the transgenic tobacco plants. These results suggest that unsaturation of fatty acids of phosphatidylglycerol in thylakoid membranes stabilizes the photosynthetic machinery against low-temperature photoinhibition by accelerating the recovery of the photosystem II protein complex.

Carotenoids, versatile components of oxygenic photosynthesis

Carotenoids (CARs) are a group of pigments that perform several important physiological functions in all kingdoms of living organisms. CARs serve as protective agents, which are essential structural components of photosynthetic complexes and membranes, and they play an important role in the light harvesting mechanism of photosynthesizing plants and cyanobacteria. The protection against reactive oxygen species, realized by quenching of singlet oxygen and the excited states of photosensitizing molecules, as well as by the scavenging of free radicals, is one of the main biological functions of CARs. X-ray crystallographic localization of CARs revealed that they are present at functionally and structurally important sites of both the PSI and PSII reaction centers. Characterization of a CAR-less cyanobacterial mutant revealed that while the absence of CARs prevents the formation of PSII complexes, it does not abolish the assembly and function of PSI. CAR molecules assist in the formation of protein subunits of the photosynthetic complexes by gluing together their protein components. In addition to their aforementioned indispensable functions, CARs have a substantial role in the formation and maintenance of proper cellular architecture, and potentially also in the protection of the translational machinery under stress conditions.

The recovery of photosynthesis from low-temperature photoinhibition is accelerated by the unsaturation of membrane lipids: a mechanism of chilling tolerance

In a previous study of mutants in fatty-acid desaturation of Synechocystis PCC6803, it was demonstrated that the photoinhibition of photosynthesis at low temperature in vivo is tolerated by cells as a result of the unsaturation of glycerolipids of thylakoid membranes. Since the extent of photoinhibition of photosynthesis in vivo depends on a balance between the photoinduced inactivation and the recovery from the photoinhibited state, an examination was made of the effects of the unsaturation of membrane lipids on these processes. It appears that the unsaturation of the membrane lipids does not affect the inactivation process but accelerates the recovery process and, moreover, that the apparent increase in the photoinhibition in vivo of photosynthesis at low temperature is caused by a depressed rate of recovery at low temperature.

Membrane dynamics as seen by fourier transform infrared spectroscopy in a cyanobacterium, Synechocystis PCC 6803. The effects of lipid unsaturation and the protein-to-lipid ratio

The roles of lipid unsaturation and lipid-protein interactions in maintaining the physiologically required membrane dynamics were investigated in a cyanobacterium strain, Synechocystis PCC 6803. The specific effects of lipid unsaturation on the membrane structure were addressed by the use of desaturase-deficient (desA(-)/desD(-)) mutant cells (which contain only oleic acid as unsaturated fatty acid species) of Synechocystis PCC 6803. The dynamic properties of the membranes were determined from the temperature dependence of the symmetric CH(2) stretching vibration frequency, which is indicative of the lipid fatty acyl chain disorder. It was found that a similar membrane dynamics is maintained at any growth temperature, in both the wild-type and the mutant cell membranes, with the exception of mutant cells grown at the lower physiological temperature limit. It seems that in the physiological temperature range the desaturase system of the cells can modulate the level of lipid desaturation sufficiently to maintain similar membrane dynamics. Below the range of normal growth temperatures, however, the extent of lipid disorder was always higher in the thylakoid than in the cytoplasmic membranes prepared from the same cells. This difference was attributed to the considerable difference in protein-to-lipid ratio in the two kinds of membranes, as determined from the ratio of the intensities of the protein amide I band and the lipid ester C&z.dbnd6;O vibration. The contributions to the membrane dynamics of an ab ovo present 'structural' lipid disorder due to the protein-lipid interactions and of a thermally induced 'dynamic' lipid disorder could be distinguished.

Direct evidence for requirement of phosphatidylglycerol in photosystem II of photosynthesis

Phosphatidylglycerol (PG) is considered to play an important role in the ordered assembly and structural maintenance of the photosynthetic apparatus in thylakoid membranes. However, its function in photosynthesis remains poorly understood. In this study we have identified a pgsA gene of Synechocystis sp. PCC6803 that encodes a PG phosphate synthase involved in the biosynthesis of PG. A disruption of the pgsA gene allowed us to manipulate the content of PG in thylakoid membranes and to investigate the function of PG in photosynthesis. The obtained pgsA mutant could grow only in the medium containing PG, and the photosynthetic activity of the pgsA mutant dramatically decreased with a concomitant decrease of PG content in thylakoid membranes when the cells grown in the presence of PG were transferred to the medium without PG. This decrease of photosynthetic activity was attributed to the decrease of photosystem (PS)II activity, but not to the decrease in PSI activity. These findings demonstrate that PG is essential for growth of Synechocystis sp. PCC6803 and provide the first direct evidence that PG plays an important role in PSII.

Unsaturation of fatty acids in membrane lipids enhances tolerance of the cyanobacterium Synechocystis PCC6803 to low-temperature photoinhibition

Effect of the unsaturation of fatty acids in the glycerolipids of thylakoid membranes on low-temperature photoinhibition of photosynthesis was studied by mutation and transformation of the cyanobacterium Synechocystis PCC6803. When grown at 34 degrees C, the wild type contained mono-, di-, and triunsaturated lipids; a mutant, designated Fad6, contained mono- and diunsaturated lipids; and a transformant of Fad6, with a disrupted gene for desaturation and designated Fad6/desA::Kmr, contained only monounsaturated lipids. Fad6/desA::Kmr was the most susceptible among these strains to low-temperature photoinhibition of photosynthesis, whereas Fad6 and the wild type were apparently indistinguishable in terms of sensitivity to photoinhibition. This result suggests that the presence of diunsaturated fatty acids is important in protecting against low-temperature photoinhibition. The photoinhibition at room temperature, although much less significant than that at low temperature, was also affected by the unsaturation of fatty acids. By contrast, the photosynthetic transport of electrons, measured at various temperatures, was not affected by changes in extent of fatty acid unsaturation.

Contribution of membrane lipids to the ability of the photosynthetic machinery to tolerate temperature stress

The contribution of the unsaturation of membrane lipids to the ability of the photosynthetic machinery to tolerate temperature stress was studied in a transgenic cyanobacterium. Anacystis nidulans R2-SPc was transformed with the desA gene, which encodes the Delta12-desaturase that desaturates the fatty acids of membrane lipids in Synechocystis PCC6803. The transformant acquired the ability to introduce a second double bond into palmitoleic and oleic acids. The transformation enhanced the tolerance of the photosynthetic machinery to chilling stress but it had no detectable effect on the ability to tolerate heat stress. The transformation itself did not have any effect on photosynthetic activity. These results imply that an increase in the unsaturation of membrane lipids enhances the tolerance of the photosynthetic machinery toward chilling stress but not toward heat stress and that such an increase does not affect photosynthesis within the range of physiological temperatures.

Phosphatidylglycerol requirement for the function of electron acceptor plastoquinone Q(B) in the photosystem II reaction center

Phosphatidylglycerol (PG), a ubiquitous constituent of thylakoid membranes of chloroplasts and cyanobacteria, is demonstrated to be essential for the functionality of plastoquinone electron acceptor Q(B) in the photosystem II reaction center of oxygenic photosynthesis. Growth of the pgsA mutant cells of Synechocystis sp. PCC6803 that are defective in phosphatidylglycerolphosphate synthase and are incapable of synthesizing PG, in a medium without PG, resulted in a 90% decrease in PG content and a 50% loss of photosynthetic oxygen-evolving activity as reported [Hagio, M., Gombos, Z., Várkonyi, Z., Masamoto, K., Sato, N., Tsuzuki, M., and Wada, H. (2000) Plant Physiol. 124, 795-804]. We have studied each step of the electron transport in photosystem II of the pgsA mutant to clarify the functional site of PG. Accumulation of Q(A)(-) was indicated by the fast rise of chlorophyll fluorescence yield under continuous and flash illumination. Oxidation of Q(A)(-) by Q(B) plastoquinone was shown to become slow, and Q(A)(-) reoxidation required a few seconds when measured by double flash fluorescence measurements. Thermoluminescence measurements further indicated the accumulation of the S(2)Q(A)(-) state but not of the S(2)Q(B)(-) state following the PG deprivation. These results suggest that the function of Q(B) plastoquinone was inactivated by the PG deprivation. We assume that PG is an indispensable component of the photosystem II reaction center complex to maintain the structural integrity of the Q(B)-binding site. These findings provide the first clear identification of a specific functional site of PG in the photosynthetic reaction center.

The action in vivo of glycine betaine in enhancement of tolerance of Synechococcus sp. strain PCC 7942 to low temperature

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.

Light-induced expression of fatty acid desaturase genes

In cyanobacterial cells, fatty acid desaturation is one of the crucial steps in the acclimation processes to low-temperature conditions. The expression of all the four acyl lipid desaturase genes of Synechocystis PCC 6803 was studied as a function of temperature and separately as a function of light. We used cells grown at 25 degreesC in light-activated heterotrophic growth conditions. In these cells, the production of alpha-linolenic acid and 18:4 fatty acids was negligible and the synthesis of gamma-linolenic acid was remarkably suppressed compared with those of the cells grown photoautotrophically. The cells grown in the light in the presence of glucose showed no difference in fatty acid composition compared with cells grown photoautotrophically. The level of desC mRNA for delta9 desaturase was not affected by either the temperature or the light. It was constitutively expressed at 25 degreesC with and without illumination. The level of desB transcripts was negligible in the dark-grown cells and was enhanced about 10-fold by exposure of the cells to light. The maximum level of expression occurred within 15 min. The level of desA and desD mRNAs was higher in dark-grown cells than that of desB mRNA for omega3 desaturase. However, the induction of both desA and desD mRNAs for delta12 and delta6 desaturases, respectively, was enhanced by light about 10-fold. Rifampicin, chloramphenicol, and 3-(3, 4-dichlorophenyl)-1,1-dimethylurea completely blocked the induction of the expression of desA, desB, and desD. Consequently, we suggest the regulatory role of light via photosynthetic processes in the induction of the expression of acyl lipid desaturases.

Non-class II HLA gene associated with type 1 diabetes maps to the 240-kb region near HLA-B

Several studies provide evidence that in addition to the DQ-DR genes, HLA contains another uncharacterized gene or genes associated with type 1 diabetes. Our aim was to investigate the effect of this gene independently of the DQ-DR genes and to localize it with a matched case-control study. More than 1,400 patients and 30,000 control individuals from Finland were studied. They were first genotyped for the selected alleles of the HLA-DQB1, -DQA1, and -DRB1 genes. For the DR3/4(0404) genotype, 75 patients and 181 control subjects were stratified, and 241 patients and 354 controls were stratified for the DR3/4(0401) genotype. Ten microsatellite markers in the HLA class III and I regions (D6S273, TNFa, C12A, STR MICA, MIB, C125, C143, C245, C3211, and MOGc) and selected alleles of the HLA-A and HLA-B genes were studied. In the DR3/4(0404)-stratified group, we found that markers located between C12A and C143 near the HLA-B gene confer a strong additional diabetes association. This was confirmed by the population differentiation test in both DR3/4(0404)- and DR3/4(0401)-stratified groups. Our data indicate that an additional gene associated with type 1 diabetes is located in the 240-kb region near HLA-B. We excluded STR MICA polymorphism as a mutation responsible for diabetes association.

Mossy fiber sprouting induced by repeated electroconvulsive shock seizures

The elicitation of repeated focal seizures (kindling) induces mossy fiber sprouting in the hippocampus of the rat. The present study investigated whether repeated generalized seizures also induce mossy fiber sprouting. Human psychiatric patients receive repeated generalized seizures during electroconvulsive therapy (ECT). Male Long-Evans rats received a course of eight electroconvulsive shock (ECS) seizures administered on a 48-h schedule over a course of 2 1/2 weeks. Control subjects received matched handling, but no stimulation. Fourteen days after the last ECS trial, all subjects were sacrificed and their brains subjected to Timm staining. Cell counts and area measures were also taken in the hilus. Significant sprouting, but not significant cell loss, was seen in the fascia dentata of the subjects that had received ECS.

Phosphatidylglycerol is essential for oligomerization of photosystem I reaction center

Our earlier studies with the pgsA mutant of Synechocystis PCC6803 demonstrated the important role of phosphatidylglycerol (PG) in PSII dimer formation and in electron transport between the primary and secondary electron-accepting plastoquinones of PSII. Using a long-term depletion of PG from pgsA mutant cells, we could induce a decrease not only in PSII but also in PSI activity. Simultaneously with the decrease in PSI activity, dramatic structural changes of the PSI complex were detected. A 21-d PG depletion resulted in the degradation of PSI trimers and concomitant accumulation of monomer PSI. The analyses of PSI particles isolated by MonoQ chromatography showed that, following the 21-d depletion, PSI trimers were no longer detectable in the thylakoid membranes. Immunoblot analyses revealed that the PSI monomers accumulating in the PG-depleted mutant cells do not contain PsaL, the protein subunit thought to be responsible for the trimer formation. Nevertheless, the trimeric structure of PSI reaction center could be restored by readdition of PG, even in the presence of the protein synthesis inhibitor lincomycin, indicating that free PsaL was present in thylakoid membranes following the 21-d PG depletion. Our data suggest an indispensable role for PG in the PsaL-mediated assembly of the PSI reaction center.

Mn and Co toxicity in chlorophyll biosynthesis

The metal ion-induced inhibition of tetrapyrrole biosynthesis was studied in the cyanobacterium Anacystis nidulans. The accumulation of protoporphyrin and Mg protoporphyrin due to the effect of Co(2+) and Mn(2+) treatment, respectively, pointed to two different sites of inhibition.

Low-temperature-induced accumulation of xanthophylls and its structural consequences in the photosynthetic membranes of the cyanobacterium Cylindrospermopsis raciborskii: an FTIR spectroscopic study

The effects of the growth temperature on the lipids and carotenoids of a filamentous cyanobacterium, Cylindrospermopsis raciborskii, were studied., The relative amounts of polyunsaturated glycerolipids and myxoxanthophylls in the thylakoid membranes increased markedly when this cyanobacterium was grown at 25 degrees C instead of 35 degrees C. Fourier transform infrared spectroscopy was used to analyze the low-temperature-induced structural alterations in the thylakoid membranes. Despite the higher amount of unsaturated lipids there, conventional analysis of the v(sym)CH(2) band (characteristic of the lipid disorder) revealed more tightly arranged fatty-acyl chains for the thylakoids in the cells grown at 25 degrees C as compared with those grown at 35 degrees C. This apparent controversy was resolved by a two-component analysis of the v(sym)CH(2) band, which demonstrated very rigid, myxoxanthophyll-related lipids in the thylakoid membranes. When this rigid component was excluded from the analysis of the thermotropic responses of the v(sym)CH(2) bands, the expected higher fatty-acyl disorder was observed for the thylakoids prepared from cells grown at 25 degrees C as compared with those grown at 35 degrees C. Both the carotenoid composition and this rigid component in the thylakoid membranes were only growth temperature-dependent; the intensity of the illuminating light during cultivation had no apparent effect on these parameters. We propose that, besides their well-known protective functions, the polar carotenoids in particular may have structural effects on the thylakoid membranes. These effects should be exerted locally--by forming protective patches, in-membrane barriers of low dynamics--to prevent the access of reactive radicals generated in either enzymatic or photosynthetic processes to sensitive spots of the membranes.

Involvement of carotenoids in the synthesis and assembly of protein subunits of photosynthetic reaction centers of Synechocystis sp. PCC 6803

The crtB gene of Synechocystis sp. PCC 6803, encoding phytoene synthase, was inactivated in the Delta crtH mutant to generate a carotenoidless Delta crtH/B double mutant. Delta crtH mutant cells were used because they had better transformability than wild-type cells, most probably due to their adaptation to partial carotenoid deficiency. Cells of the Delta crtH/B mutant were light sensitive and could grow only under light-activated heterotrophic growth conditions in the presence of glucose. Carotenoid deficiency did not significantly affect the cellular content of phycobiliproteins while the chlorophyll content of the mutant cells decreased. The mutant cells exhibited no oxygen-evolving activity, suggesting the absence of photochemically active PSII complexes. This was confirmed by 2D electrophoresis of photosynthetic membrane complexes. Analyses identified only a small amount of a non-functional PSII core complex lacking CP43, while the monomeric and dimeric PSII core complexes were absent. On the other hand, carotenoid deficiency did not prevent formation of the cytochrome b(6)f complex and PSI, which predominantly accumulated in the monomeric form. Radioactive labeling revealed very limited synthesis of inner PSII antennae, CP47 and especially CP43. Thus, carotenoids are indispensable constituents of the photosynthetic apparatus, being essential not only for antioxidative protection but also for the efficient synthesis and accumulation of photosynthetic proteins and especially that of PSII antenna subunits.

The Unsaturation of Membrane Lipids Stabilizes Photosynthesis against Heat Stress

The effect of the unsaturation of glycerolipids of thylakoid membranes on the heat tolerance of the photosynthetic evolution of oxygen was studied in vivo by mutation and transformation of fatty-acid desaturases in the cyanobacterium Synechocystis PCC6803. The experimental results indicate that elimination of dienoic lipid molecules decreases, to a small but distinct extent, the heat tolerance of photosynthetic oxygen evolution, but that elimination of trienoic lipid molecules has no effect on the heat tolerance. This conclusion contrasts with the previous hypothesis that the heat tolerance of photosynthesis is enhanced upon an increase in the level of saturation of membrane lipids. It is also shown that light does not affect the nature of the effect of lipid unsaturation on the heat tolerance of photosynthesis.

Genetic Enhancement of the Ability to Tolerate Photoinhibition by Introduction of Unsaturated Bonds into Membrane Glycerolipids

Strong light leads to damage to photosynthetic machinery, particularly at low temperatures, and the main site of the damage is the D1 protein of the photosystem II (PSII) complex. Here we describe that transformation of Synechococcus sp. PCC 7942 with the desA gene for a [delta]12 desaturase increased unsaturation of membrane lipids and enhanced tolerance to strong light. To our knowledge, this is the first report of the successful genetic enhancement of tolerance to strong light. Analysis of the light-induced inactivation and of the subsequent recovery of the activity of the PSII complex revealed that the recovery process was markedly accelerated by the genetic transformation. Labeling experiments with [35S]L-methionine also revealed that the synthesis of the D1 protein de novo at low temperature, which was a prerequisite for the restoration of the PSII complex, was much faster in the transformed cells than in the wild-type cells. These findings demonstrate that the ability of membrane lipids to desaturate fatty acids is important for the photosynthetic organisms to tolerate strong light, by accelerating the synthesis of the D1 protein de novo.

Immunocytochemical localization of acyl-lipid desaturases in cyanobacterial cells: evidence that both thylakoid membranes and cytoplasmic membranes are sites of lipid desaturation

There are four acyl-lipid desaturases in the cyanobacterium Synechocystis sp. PCC 6803. Each of these desaturases introduces a double bond at a specific position, such as the Delta6, Delta9, Delta12, or omicron3 position, in C18 fatty acids. The localization of the desaturases in cyanobacterial cells was examined immunocytochemically with antibodies raised against synthetic oligopeptides that corresponded to the carboxyl-terminal regions of the desaturases. All four desaturases appeared to be located in the regions of both the cytoplasmic and the thylakoid membranes. These findings suggest that fatty acid desaturation of membrane lipids takes place in the thylakoid membranes as well as in the cytoplasmic membranes.

Analysis of extended human leukocyte antigen haplotype association with Addison's disease in three populations

OBJECTIVE

Addison's disease is an organ-specific autoimmune disorder with a polygenic background. The aim of the study was to identify non-class II human leukocyte antigen (HLA) susceptibility genes for Addison's disease.

DESIGN AND METHODS

Addison's disease patients from three European populations were analysed for selected HLA-DR-DQ alleles and for 11 microsatellite markers covering approximately 4 Mb over the HLA region. Subjects were 69 patients with Addison's disease from Estonia (24), Finland (14) and Russia (31). Consecutively recruited healthy newborns from the same geographical regions were used as controls (269 Estonian, 1000 Finnish and 413 Russian). Association measures for HLA-DRB1, DQB1, DQA1 and 11 microsatellites between D6S273 and D6S2223 were taken. A low-resolution full-house typing was used for HLA class II genes, while microsatellite markers were studied using fluorescence-based DNA fragment sizing technology.

RESULTS

We confirmed that the HLA-DR3-DQ2 and the DQB1*0302-DRB1*0404 haplotypes confer disease susceptibility. In Russian patients, we also found an increase of DRB1*0403 allele, combined with DQB1*0305 allele in three out of six cases (P<0.0001). Analysis of 11 microsatellite markers including STR MICA confirmed the strong linkage in DR3-DQ2 haplotypes but DRB1*0404-DQB1*0302 haplotypes were diverse. MICA5.1 allele was found in 22 out of 24 Estonian patients, but results from Finnish and Russian patients did not support its independent role in disease susceptibility.

CONCLUSION

HLA-DRB1*0403 was identified as a novel susceptibility allele for Addison's disease. Additionally, we found no evidence of a non-class II HLA disease susceptibility locus; however, the HLA-DR3-DQ2 haplotype appeared more conserved in patient groups with high DR-DQ2 frequencies.

Heat- and light-induced reorganizations in the phycobilisome antenna of Synechocystis sp. PCC 6803. Thermo-optic effect

By using absorption and fluorescence spectroscopy, we compared the effects of heat and light treatments on the phycobilisome (PBS) antenna of Synechocystis sp. PCC 6803 cells. Fluorescence emission spectra obtained upon exciting predominantly PBS, recorded at 25 degrees C and 77 K, revealed characteristic changes upon heat treatment of the cells. A 5-min incubation at 50 degrees C, which completely inactivated the activity of photosystem II, led to a small but statistically significant decrease in the F(680)/F(655) fluorescence intensity ratio. In contrast, heat treatment at 60 degrees C resulted in a much larger decrease in the same ratio and was accompanied by a blue-shift of the main PBS emission band at around 655 nm (F(655)), indicating an energetic decoupling of PBS from chlorophylls and reorganizations in its internal structure. (Upon exciting PBS, F(680) originates from photosystem II and from the terminal emitter of PBS.). Very similar changes were obtained upon exposing the cells to high light (600-7500 micromol photons m(-2) s(-1)) for different time periods (10 min to 3 h). In cells with heat-inactivated photosystem II, the variations caused by light treatment could clearly be assigned to a similar energetic decoupling of the PBS from the membrane and internal reorganizations as induced at around 60 degrees C. These data can be explained within the frameworks of thermo-optic mechanism [Cseh et al. 2000, Biochemistry 39, 15250]: in high light the heat packages originating from dissipation might lead to elementary structural changes in the close vicinity of dissipation in heat-sensitive structural elements, e.g. around the site where PBS is anchored to the membrane. This, in turn, brings about a diminishment in the energy supply from PBS to the photosystems and reorganization in the molecular architecture of PBS.

Characterization of the Fad12 mutant of Synechocystis that is defective in delta 12 acyl-lipid desaturase activity

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.