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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 role of unsaturated fatty acids in membrane lipids in the tolerance of the photosynthetic machinery to salt stress was studied by comparing the desA ؊ ͞desD ؊ mutant of Synechocystis sp. PCC 6803, which contained monounsaturated fatty acids, with the wild-type strain, which contained a full complement of polyunsaturated fatty acids. In darkness, the loss of oxygen-evolving photosystem II activity in the presence of 0.5 M NaCl or 0.5 M LiCl was much more rapid in desA ؊ ͞desD ؊ cells than in wild-type cells. Oxygenevolving activity that had been lost during incubation with 0.5 M NaCl in darkness returned when cells were transferred to conditions that allowed photosynthesis or respiration. Recovery was much greater in wild-type than in desA ؊ ͞desD ؊ cells, and it was prevented by lincomycin. Thus, the unsaturation of fatty acids is important in the tolerance of the photosynthetic machinery to salt stress. It appears also that the activity and synthesis of the Na ؉ ͞H ؉ antiporter system might be suppressed under high-salt conditions and that this effect can be reversed, in part, by the unsaturation of fatty acids in membrane lipids.The regulation of the unsaturation of fatty acids in membrane lipids is important in the acclimation of poikilothermic organisms to changing environmental conditions, in particular, temperature (1-4). The effects of temperature on the desaturation of fatty acids have been extensively studied in cyanobacteria (for reviews, see refs. 2 and 5). When the ambient temperature is shifted from high to low, cyanobacterial cells synthesize fatty acid desaturases that introduce double bonds into the fatty acids of membrane lipids.In a previous study we isolated a desA Ϫ ͞desD Ϫ mutant strain of the cyanobacterium Synechocystis sp. PCC 6803 in which the desA and desD genes for the ⌬12 and ⌬6 desaturases, respectively, had been inactivated by targeted mutagenesis (6). The desA Ϫ ͞desD Ϫ cells contain monounsaturated but not polyunsaturated fatty acids, whereas wild-type cells contain polyunsaturated fatty acids such as di-, tri-, and tetraunsaturated fatty acids (7). By comparing these two strains we found that polyunsaturated fatty acids are essential for growth in Synechocystis sp. PCC 6803 at low temperatures (6).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 Ϫ ͞desD Ϫ 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. MATERIALS AND METHODS Cells, Culture Conditions, and Preparation of ThylakoidMembranes. Wild-type and desA Ϫ ͞desD Ϫ (6) cells of Synechocystis sp. PCC 6803 were grown photoautotrophically in BG-11 medium (pH 7.5) (23), which contained 20 mM Na ϩ ions at 35°C under constant illumination from incandescent lamps (70-80 E⅐m Ϫ2 ⅐s Ϫ1, in which E indicates an einstein, 1 mol of photons) with aeration by sterile air that cont...