Photoinhibition in mutants of Arabidopsis deficient in thylakoid unsaturation

Thylakoid lipid composition in higher plants is characterized by a high level of fatty acid unsaturation. We have screened four mutants of Arabidopsis that have reduced levels of fatty acid unsaturation. Three of the mutant lines tested, fad5, fad6, and the fad3-2 fad7-2 fad8 triple mutant, were more susceptible to photoinhibition than wild-type Arabidopsis, whereas one mutant, fab1, was indistinguishable from wild type. The fad3-2 fad7-2 fad8 triple mutant, which contains no trienoic fatty acids in its thylakoid membranes, was most susceptible to photoinhibition. Detailed investigation of photoinhibition in the triple mutant revealed that the rate of photoinactivation of PSII was the same in wild-type and mutant plants. However, the recovery of photoinactivated PSII was slower in fad3-2 fad7-2 fad8, relative to wild type, at all temperatures below 27 degrees C. These results indicate that trienoic fatty acids of thylakoid membrane lipids are required for low-temperature recovery from photoinhibition in Arabidopsis.     

Localisation and processing of the precursor form of photosystem II protein D1 inSynechocystis 6803

Pure plasma membrane and thylakoid membrane fractions from Synechocystis 6803 were isolated to study the localisation and processing of the precursor form of the D1 protein (pD1) of photosystem II (PSII). PSII core proteins (D1, D2 and cytb559) were localised both to plasma and thylakoid membrane fractions, the majority in thylakoids. pD1 was found only in the thylakoid membrane where active PSII is known to function. Membrane fatty acid unsaturation was shown to be critical in processing of pD1 into mature D1 protein. This was concluded from pulse-labelling experiments at low temperature using wild type and a mutant Synechocystis 6803 with a low level of membrane fatty acid unsaturation. Further, pD1 was identified as two distinct bands, an indication of two cleavage sites in the precursor peptide or, alternatively, two different conformations of pD1. Our results provide evidence for thylakoid membranes being a primary synthesis site for D1 protein during its light-activated turnover. The existence of the PSII core proteins in the plasma membrane, on the other hand, may be related to the biosynthesis of new PSII complexes in these membranes.     

Irreversible photoinhibition of photosystem II is caused by exposure of Synechocystis cells to strong light for a prolonged period

Irreversible photoinhibition of photosystem II (PSII) occurred when Synechocystis sp. PCC 6803 cells were exposed to very strong light for a prolonged period. When wild-type cells were illuminated at 20 degrees C for 2 h with light at an intensity of 2,500 micromol photons m(-2) s(-1), the oxygen-evolving activity of PSII was almost entirely and irreversibly lost, whereas the photochemical reaction center in PSII was inactivated only reversibly. The extent of irreversible photoinhibition was enhanced at lower temperatures and by the genetically engineered rigidification of membrane lipids. Western and Northern blotting demonstrated that, after cells had undergone irreversible photoinhibition, the precursor to D1 protein in PSII was synthesized but not processed properly. These observations may suggest that exposure of Synechocystis cells to strong light results in the irreversible photoinhibition of the oxygen-evolving activity of PSII via impairment of the processing of pre-D1 and that this effect of strong light is enhanced by the rigidification of membrane lipids.     

Phase Transition of Thylakoid Membranes Modulates Photoinhibition in the Cyanobacterium Anabaena siamensis

A shift of the growth temperature from 40 degrees C to 18 degrees C promoted an increase in the degree of fatty acids unsaturation and a decrease, from 26 degrees C to 0 degrees C, of the phase transition temperature of thylakoid membranes in Anabaena siamensis. The pattern of photoinhibition of photosynthesis at distinct temperatures varied as a function of the phase transition temperature. In the absence of streptomycin, a pronounced photoinhibition at temperatures near the phase transition (26 degrees C) was observed in cells grown at 40 degrees C, while protection from photodamage was observed at chilling temperatures (15 degrees C to 5 degrees C). In this same range of temperature, such a protection was not verified if cells were grown at 18 degrees C. In both types of cells, however, the rate of photoinactivation in the presence of streptomycin was progressively decreased by lowering the temperature of photoinhibition. When recovery from photoinhibition was followed at the respective temperature in which cells were grown, the restoration profile of the photosynthetic O(2) evolution to initial levels was essentially the same in both types of cells. The protective effect of low temperatures against photoinhibition was attributed to a decreased solubility and diffusion of oxygen in the thylakoid membranes due to an increase of the membrane viscosity that would avoid the photogeneration of reactive oxygen species around PS II.     

LPA2 is required for efficient assembly of photosystem II in Arabidopsis thaliana

To elucidate the molecular mechanism of photosystem II (PSII) assembly, we characterized the low psii accumulation2 (lpa2) mutant of Arabidopsis thaliana, which is defective in the accumulation of PSII supercomplexes. The levels and processing patterns of the RNAs encoding the PSII subunits are unaltered in the mutant. In vivo protein-labeling experiments showed that the synthesis of CP43 (for chlorophyll a binding protein) was greatly reduced, but CP47, D1, and D2 were synthesized at normal rates in the lpa2-1 mutant. The newly synthesized CP43 was rapidly degraded in lpa2-1, and the turnover rates of D1 and D2 were higher in lpa2-1 than in wild-type plants. The newly synthesized PSII proteins were assembled into PSII complexes, but the assembly of PSII was less efficient in the mutant than in wild-type plants. LPA2 encodes an intrinsic thylakoid membrane protein, which is not an integral subunit of PSII. Yeast two-hybrid assays indicated that LPA2 interacts with the PSII core protein CP43 but not with the PSII reaction center proteins D1 and D2. Moreover, direct interactions of LPA2 with Albino3 (Alb3), which is involved in thylakoid membrane biogenesis and cell division, were also detected. Thus, the results suggest that LPA2, which appears to form a complex with Alb3, is involved in assisting CP43 assembly within PSII.     

Irreversible photoinhibition of photosystem II is caused by exposure of Synechocystis cells to strong light for a prolonged period

Irreversible photoinhibition of photosystem II (PSII) occurred when Synechocystis sp. PCC 6803 cells were exposed to very strong light for a prolonged period. When wild-type cells were illuminated at 20 °C for 2 h with light at an intensity of 2,500 μmol photons m⁻² s⁻¹, the oxygen-evolving activity of PSII was almost entirely and irreversibly lost, whereas the photochemical reaction center in PSII was inactivated only reversibly. The extent of irreversible photoinhibition was enhanced at lower temperatures and by the genetically engineered rigidification of membrane lipids. Western and Northern blotting demonstrated that, after cells had undergone irreversible photoinhibition, the precursor to D1 protein in PSII was synthesized but not processed properly. These observations may suggest that exposure of Synechocystis cells to strong light results in the irreversible photoinhibition of the oxygen-evolving activity of PSII via impairment of the processing of pre-D1 and that this effect of strong light is enhanced by the rigidification of membrane lipids.     

Carboxyl-terminal processing of the D1 protein and photoactivation of water-splitting in photosystem II. Partial purification and characterization of the processing enzyme from Scenedesmus obliquus and Pisum sativum.

The D1 polypeptide of photosystem II (PSII) is synthesized as a precursor that is processed by cleavage at the carboxyl terminus during assembly of the active PSII complex. A mutant of the green alga Scenedesmus obliquus, LF-1, inactive in water-splitting, lacks the D1 processing activity but assembles otherwise normal PSII complexes containing the precursor D1 molecule. We have isolated and partially purified a soluble protease from sonicated thylakoids of both wild-type S. obliquus and Pisum sativum which will process the precursor D1 molecule in PSII-enriched membranes from the LF-1 mutant to the mature size. After processing (but not before), photoactivation of these PSII membranes in the presence of manganese restores water-splitting to levels seen after photoactivation of PSII membranes from dark-grown, wild-type, cells. The protease is unable to process D1 in intact thylakoids from the LF-1 mutant but processes D1 if present during sonication of the thylakoids, indicating that processing of the carboxyl-terminal extension of D1 occurs in the lumen of the thylakoid. The processing protease from both S. obliquus and P. sativum is a single subunit enzyme of native molecular mass 33-35 kDa. Processing rate is optimal at pH 6.5. Processing in vitro is evident within 5 min and is markedly inhibited by millimolar concentrations of divalent cations (Cu, Zn greater than Mn greater than Ca, Mg) but not by any known inhibitors of the major classes of proteases. The protease is inactive against the precursors of other thylakoidal proteins and is thus distinct from the thylakoidal amino-terminal processing enzyme involved in the removal of transit peptides from cytoplasmically-synthesised proteins imported into the thylakoid lumen.     

Application of low temperatures during photoinhibition allows characterization of individual steps in photodamage and the repair of photosystem II

Recent investigations of photoinhibition have revealed that photodamage to photosystem II (PSII) involves two temporally separated steps: the first is the inactivation of the oxygen-evolving complex by light that has been absorbed by the manganese cluster and the second is the impairment of the photochemical reaction center by light that has been absorbed by chlorophyll. Our studies of photoinhibition in Synechocystis sp. PCC 6803 at various temperatures demonstrated that the first step in photodamage is not completed at low temperatures, such as 10°C. Further investigations suggested that an intermediate state, which is stabilized at low temperatures, might exist at the first stage of photodamage. The repair of PSII involves many steps, including degradation and removal of the D1 protein, synthesis de novo of the precursor to the D1 protein, assembly of the PSII complex, and processing of the precursor to the D1 protein. Detailed analysis of photodamage and repair at various temperatures has demonstrated that, among these steps, only the synthesis of the precursor to D1 appears to proceed at low temperatures. Investigations of photoinhibition at low temperatures have also indicated that prolonged exposure of cyanobacterial cells or plant leaves to strong light diminishes their ability to repair PSII. Such non-repairable photoinhibition is caused by inhibition of the processing of the precursor to the D1 protein after prolonged illumination with strong light at low temperatures.     

Involvement of zeaxanthin and of the Cbr protein in the repair of photosystem II from photoinhibition in the green alga Dunaliella salina.

A light-sensitive and chlorophyll (Chl)-deficient mutant of the green alga Dunaliella salina (dcd1) showed an amplified response to irradiance stress compared to the wild-type. The mutant was yellow-green under low light (100 micromol photons m(-2) s(-1)) and yellow under high irradiance (2000 micromol photons m(-2) s(-1)). The mutant had lower levels of Chl, lower levels of light harvesting complex II, and a smaller Chl antenna size. The mutant contained proportionately greater amounts of photodamaged photosystem (PS) II reaction centers in its thylakoid membranes, suggesting a greater susceptibility to photoinhibition. This phenotype was more pronounced under high than low irradiance. The Cbr protein, known to accumulate when D. salina is exposed to irradiance stress, was pronouncedly expressed in the mutant even under low irradiance. This positively correlated with a higher zeaxanthin content in the mutant. Cbr protein accumulation, xanthophyll cycle de-epoxidation state, and fraction of photodamaged PSII reaction centers in the thylakoid membrane showed a linear dependence on the chloroplast 'photoinhibition index', suggesting a cause-and-effect relationship between photoinhibition, Cbr protein accumulation and xanthophyll cycle de-epoxidation state. These results raised the possibility of zeaxanthin and Cbr involvement in the PSII repair process through photoprotection of the partially disassembled, and presumably vulnerable, PSII core complexes from potentially irreversible photooxidative bleaching.     

Changes in Unsaturated Levels of Fatty Acids in Thylakoid PSII Membrane Lipids During Chilling-induced Resistance in Rice

Temperature is one of the abiotic factors limiting growth and productivity of plants. In the present work, the effect of low non‐freezing temperature, as an inducer of “chilling resistance”, was studied in three cultivars of rice (Oryza sativa L.), japonica cv. 9516 (j‐9516), the two parental lines of superhigh‐yield hybrid rice between subspecies, Peiai/E32 (ji‐PE), and the traditional indica hybrid rice Shanyou 63 (i‐SY63). Leaves of chill‐treated rice showed chilling‐induced resistance, as an increase of their low‐temperature tolerance was measured using chlorophyll fluorescence measurements, revealing a change in photosystem II (PSII) efficiency. After 5 d of exposure to 11°C under low light (100 μmol m^‐2 s^‐1), levels of unsaturated fatty acids in PSII thylakoid membrane lipids decreased during the initial 1‐2 d, then increased slowly and reached 99.2%, 95.3% and 90.1% of the initial value (0 d) in j‐9516, ji‐PE and i‐SY63, respectively, on the fifth day. However, under medium light (600 μmol m^‐2 s^‐1), all cultivars experienced similar substantial photoinhibition, which approached steady state levels after a decline in levels of unsaturated fatty acids in PSII thylakoid membrane lipids to about 57.1%, 53.8% and 44.5% of the initial values (0 d) in j‐9516, ji‐PE and I‐SY63 on the fifth day. Under either chilling‐induced resistance (the former) or low temperature photoinhibition (the latter) conditions, the changes of other physiological parameters such as D1 protein contents, electron transport activities of PSII (ETA), F_v/F_m, xanthophyl cycle activities expressed by DES (deepoxide state) were consistent with that of levels of unsaturated fatty acids in PSII thylakoid membrane lipids. So there were negative correlations between saturated levels of fatty acids (16:1(3t), 16:0, 18:0), especially the 16:1(3t) fatty acid on thylakoid membrane and other physiological parameters, such as D1 protein contents, ETA and (A+Z)/(A+V+Z). A specific role of desaturation of fatty acids and the photoprotective pigments of the xanthophyl cycle, leading to an acclimation response in thylakoid membrane lipids may be involved. We conclude that chilling‐induced resistance is accelerated by the unsaturation of thylakoid membranes, and the ability of rice plants to cold‐harden can be enhanced by genetic engineering.     

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.     

PratA, a periplasmic tetratricopeptide repeat protein involved in biogenesis of photosystem II in Synechocystis sp. PCC 6803

The light reactions of oxygenic photosynthesis are mediated by multisubunit pigment-protein complexes situated within the specialized thylakoid membrane system. The biogenesis of these complexes is regulated by transacting factors that affect the expression of the respective subunit genes and/or the assembly of their products. Here we report on the analysis of the PratA gene from the cyanobacterium Synechocystis sp. PCC 6803 that encodes a periplasmic tetratricopeptide repeat protein of formerly unknown function. Targeted inactivation of PratA resulted in drastically reduced photosystem II (PSII) content. Protein pulse labeling experiments of PSII subunits indicated that the C-terminal processing of the precursor of the reaction center protein D1 is compromised in the pratA mutant. Moreover, a direct interaction of PratA and precursor D1 was demonstrated by applying yeast two-hybrid analyses. This suggests that PratA represents a factor facilitating D1 maturation via the endoprotease CtpA. The periplasmic localization of PratA supports a model that predicts the initial steps of PSII biogenesis to occur at the plasma membrane of cyanobacterial cells.     

Requirement of phosphatidylglycerol for maintenance of photosynthetic machinery

Phosphatidylglycerol (PG) is a ubiquitous component of thylakoid membranes. Experiments with the pgsA mutant of the cyanobacterium Synechocystis sp. PCC6803 defective in biosynthesis of PG have demonstrated an indispensable role of PG in photosynthesis. In the present study, we have investigated the light susceptibility of the pgsA mutant with regard to the maintenance of the photosynthetic machinery. Growth of the mutant cells without PG increased the light susceptibility of the cells and resulted in severe photoinhibition of photosynthesis upon a high-light treatment, whereas the growth in the presence of PG was protected against photoinhibition. Photoinhibition induced by PG deprivation was mainly caused by an impairment of the restoration process. The primary target of the light-induced damage in thylakoid membranes, the D1 protein of photosystem (PS) II was, however, synthesized and degraded with similar rates irrespective of whether the mutant cells were incubated with PG or not. Intriguingly, it was found that instead of the synthesis of the D1 protein, the dimerization of the PSII core monomers was impaired in the PG-deprived mutant cells. Addition of PG to photoinhibited cells restored the dimerization capacity of PSII core monomers. These results suggest that PG plays an important role in the maintenance of the photosynthetic machinery through the dimerization and reactivation of the PSII core complex.     

Changes in Unsaturated Levels of Fatty Acids in Thylakoid PSⅡ Membrane Lipids During Chilling-induced Resistance in Rice

Temperature is one of the abiotic factors limiting growth and productivity of plants. In the present work, the effect of low non-freezing temperature, as an inducer of "chilling resistance", was studied in three cultivars of rice (Oryza sativa L.), japonica cv. 9516 (j-9516), the two parental lines of superhigh-yield hybrid rice between subspecies,Peiai/E32 (ji-PE), and the traditional indica hybrid rice Shanyou 63 (i-SY63). Leaves of chill-treated rice showed chilling-induced resistance, as an increase of their low-temperature tolerance was measured using chlorophyll fluorescence measurements, revealing a change in photosystem Ⅱ (PSⅡ) efficiency. After 5 d of exposure to 11℃ under low light (100 μ mol·m-2·s-1), levels of unsaturated fatty acids in PSⅡ thylakoid membrane lipids decreased during the initial 1-2 d, then increased slowly and reached 99.2％, 95.3％ and 90.1％ of the initial value (0 d) in j-9516,ji-PE and i-SY63, respectively, on the fifth day. However, under medium light (600 μmol·m-2·s-1), all cultivars experienced similar substantial photoinhibition, which approached steady state levels after a decline in levels of unsaturated fatty acids in PSII thylakoid membrane lipids to about 57.1％, 53.8％ and 44.5％ of the initial values (0 d) in j-9516,ji-PE and i-SY63 on the fifth day. Under either chilling-induced resistance (the former) or low temperature photoinhibition (the latter) conditions, the changes of other physiological parameters such as D1 protein contents,electron transport activities of PSII (ETA), Fv/Fm, xanthophyl cycle activities expressed by DES (deepoxide state)were consistent with that of levels of unsaturated fatty acids in PSⅡ thylakoid membrane lipids. So there were negative correlations between saturated levels of fatty acids (16:1(3t), 16:0, 18:0), especially the 16:1(3t) fatty acid on thylakoid membrane and other physiological parameters, such as D1 protein contents, ETA and (A+Z)/(A+V+Z). A specific role of desaturation of fatty acids and the photoprotective pigments of the xanthophyl cycle, leading to an acclimation response in thylakoid membrane lipids may be involved. We conclude that chilling-induced resistance is accelerated by the unsaturation of thylakoid membranes, and the ability of rice plants to cold-harden can be enhanced by genetic engineering.     

Altered rates of protein transport in Arabidopsis mutants deficient in chloroplast membrane unsaturation

Protein transfer across membranes is mediated by protein machinery embedded in the membrane. The complement of different lipid classes within a membrane is known to influence the efficiency of some protein translocation processes, but very little is known about whether the fatty acid composition of the membrane bilayer also affects protein transport. We investigated this issue using three mutants of Arabidopsis, fad6, fad5, and fad3 fad7 fad8, that have reduced levels of fatty acid unsaturation in their thylakoid membranes. Interestingly, the effect of reduced unsaturation was different for three distinct pathways of protein transport. In thylakoids from all three mutants, transport of the OE17 protein on the DeltapH/Tat pathway was reduced by up to 50% relative to wild-type controls, when assays were run at 10, 20 or 30 degrees C. By contrast, transport of the OE33 protein on the Sec pathway was substantially increased in all the mutants at the three temperatures. Transport of the CF(O)II protein (ATPg) on the 'spontaneous' pathway was largely unaffected by reduced unsaturation of the thylakoid membranes. Experiments with intact chloroplasts from wild-type Arabidopsis and the three mutants confirmed these changes in thylakoid transport reactions and also demonstrated an increased rate of protein import across the chloroplast envelope in each of the mutants. This increased capacity of chloroplast protein import may partially compensate for the reduced capacity of thylakoid transport by the DeltapH/Tat pathway. The fad5, fad6 and fad3 fad7 fad8 mutants used in this study grow normally at 15-20 degrees C, but exhibit reduced photosynthesis and growth, relative to wild-type controls, at low temperatures (4 degrees C). The results reported here indicate that protein transport and chloroplast biogenesis may well contribute to these low-temperature phenotypes.     

Role of the PsbI protein in photosystem II assembly and repair in the cyanobacterium Synechocystis sp. PCC 6803

The involvement of the PsbI protein in the assembly and repair of the photosystem II (PSII) complex has been studied in the cyanobacterium Synechocystis sp. PCC 6803. Analysis of PSII complexes in the wild-type strain showed that the PsbI protein was present in dimeric and monomeric core complexes, core complexes lacking CP43, and in reaction center complexes containing D1, D2, and cytochrome b-559. In addition, immunoprecipitation experiments and the use of a histidine-tagged derivative of PsbI have revealed the presence in the thylakoid membrane of assembly complexes containing PsbI and either the precursor or mature forms of D1. Analysis of PSII assembly in the psbI deletion mutant and in strains lacking PsbI together with other PSII subunits showed that PsbI was not required for formation of PSII reaction center complexes or core complexes, although levels of unassembled D1 were reduced in its absence. However, loss of PsbI led to a dramatic destabilization of CP43 binding within monomeric and dimeric PSII core complexes. Despite the close structural relationship between D1 and PsbI in the PSII complex, PsbI turned over much slower than D1, whereas high light-induced turnover of D1 was accelerated in the absence of PsbI. Overall, our results suggest that PsbI is an early assembly partner for D1 and that it plays a functional role in stabilizing the binding of CP43 in the PSII holoenzyme.     

Formation of DEG5 and DEG8 complexes and their involvement in the degradation of photodamaged photosystem II reaction center D1 protein in Arabidopsis

The widely distributed DEGP proteases play important roles in the degradation of damaged and misfolded proteins. Arabidopsis thaliana contains 16 DEGP-like proteases, four of which are located in the chloroplast. Here, we show that DEG5 and DEG8 form a hexamer in the thylakoid lumen and that recombinant DEG8 is proteolytically active toward both a model substrate (beta-casein) and photodamaged D1 protein of photosystem II (PSII), producing 16-kD N-terminal and 18-kD C-terminal fragments. Inactivation of DEG5 and DEG8 resulted in increased sensitivity to photoinhibition. Turnover of newly synthesized D1 protein in the deg5 deg8 double mutant was impaired, and the degradation of D1 in the presence of the chloroplast protein synthesis inhibitor lincomycin under high-light treatment was slowed in the mutants. Thus, DEG5 and DEG8 are important for efficient turnover of the D1 protein and for protection against photoinhibition in vivo. The deg5 deg8 double mutant showed increased photosensitivity and reduced rates of D1 degradation compared with single mutants of deg5 and deg8. A 16-kD N-terminal degradation fragment of the D1 protein was detected in wild-type plants but not in the deg5 deg8 mutant following in vivo photoinhibition. Therefore, our results suggest that DEG5 and DEG8 have a synergistic function in the primary cleavage of the CD loop of the PSII reaction center protein D1.     

Changes in Thermostability of Photosystem Ⅱ and Leaf Lipid Composition of Rice Mutant with Deficiency of Light-harvesting Chlorophyll a/b Protein Complexes

We studied the difference in thermostability of photosystem Ⅱ （PSII） and leaf lipid composition between a T-DNA insertion mutant rice （Oryza sativa L.） VG28 and its wild type Zhonghuau. Native green gel and SDS-PAGE electrophoreses revealed that the mutant VG28 lacked all light-harvesting chlorophyll a/b protein complexes. Both the mutant and wild type were sensitive to high temperatures, and the maximal efficiency of PSII photochemistry （FJ Fm） and oxygen-evolving activity of PSII in leaves significantly decreased with increasing temperature. However, the PSII activity of the mutant was markedly more sensitive to high temperatures than that of the wild type. Lipid composition analysis showed that the mutant had less phosphatidylglycerol and sulfoquinovosyl diacylglycerol compared with the wild type. Fatty acid analysis revealed that the mutant had an obvious decrease in the content of 16：1t and a marked increase in the content of 18：3 compared with the wild type. The effects of lipid composition and unsaturation of membrane lipids on the thermostability of PSII are discussed.     

Acclimation to the growth temperature and thermosensitivity of photosystem II in a mesophilic cyanobacterium, Synechocystis sp. PCC6803

Differences in the temperature dependence and thermosensitivities of PSII activities in Synechocystis sp. PCC6803 grown at 25 and 35 degrees C were studied. Hill reactions in cells, thylakoid membranes and purified PSII core complexes were measured at high temperatures or at their growth temperatures after high-temperature treatments. In the presence of 2,5-dichloro-p-benzoquinone as an electron acceptor, which can accept electrons directly from Q(A), the temperature dependence of the oxygen-evolving activity was almost the same in thylakoid membranes and in the purified PSII complexes from cells grown at 25 or 35 degrees C. When duroquinone, which accepts electrons only through Q(B) plastoquinone, was used as an electron acceptor, the temperature dependence was the same for purified PSII core complexes but was different between thylakoids isolated from the cells grown at 25 and 35 degrees C. No remarkable difference was observed in protein compositions between thylakoids and between purified PSII complexes from cells grown at 25 or 35 degrees C. However, the fluidity of thylakoids, measured by electron flow to P700, was affected by the growth temperature. These results suggest that one of the major factors which cause the changes in the thermosensitivity of PSII is the change in the fluidity of thylakoid membranes. As for the acclimation of PSII in thylakoids to high temperatures, one of the main causes is the decrease in the high-temperature-induced formation of non-Q(B) PSII due to the decreased fluidity in the cells grown at 35 degrees C.     

Synthesis of fatty acids de novo is required for photosynthetic acclimation of Synechocystis sp. PCC 6803 to high temperature

The role of fatty acid synthesis in the acclimation of the photosynthetic machinery to high temperature was investigated in a mutant of the cyanobacterium Synechocystis sp. PCC 6803 that had a lower than wild-type level of enoyl-(acyl-carrier-protein) reductase FabI, a key component of the type-II fatty acid synthase system. The mutant exhibited marked impairment in the tolerance and acclimation of cells to high temperature: photoautotrophic growth of the mutant was severely inhibited at 40 °C. Moreover, mutant cells were unable to achieve wild-type enhancement of the thermal stability of photosystem II (PSII) when the growth temperature was raised from 25 °C to 38 °C. Enhancement of the thermal stability of PSII was abolished when wild-type cells were treated with triclosan, a specific inhibitor of FabI, and the enhancement of thermal stability was also blocked in darkness and in the presence of chloramphenicol. Analysis of fatty acids in thylakoid membranes revealed that levels of unsaturated fatty acids did not differ between mutant and wild-type cells, indicating that the saturation of fatty acids in membrane lipids might not be responsible for the enhancement of thermal stability at elevated temperatures. Our observations suggest that the synthesis de novo of fatty acids, as well as proteins, is required for the enhancement of the thermal stability of PSII during the acclimation of Synechocystis cells to high temperature.