A genetically engineered increase in fatty acid unsaturation in Synechococcus sp. PCC 7942 allows exchange of D1 protein forms and sustenance of photosystem II activity at low temperature.

Photosystem II (PSII)-reaction-center protein D1 is encoded by three psbA genes in Synechococcus sp. PCC 7942. The psbAI gene encodes D1 form I (D1:1) and the psbAII and psbAIII genes encode the transiently expressed D1 form II (D1:2). We have studied the role of membrane-lipid unsaturation in the expression of psbA genes at low temperature, using a Synechococcus transformant with an increased unsaturation level of membrane lipids. Transfer of the cells from 32 degrees C to 25 degrees C under growth light resulted in the exchange of D1:1, the prevailing form, for D1:2 in the wild-type bacterium and the transformant, with no loss of PSII activity. Lowering the temperature further to 18 degrees C caused a drastic decrease in PSII activity in the wild-type bacterium, whereas the transformant was much less affected. Similar decreases in psbAI transcripts and loss of D1:1 occurred in both strains at 18 degrees C, with concomitant accumulation of psbAII/III transcripts, the latter event being especially prominent in the wild-type bacterium. However, the wild-type bacterium was incapable of accumulating D1:2 to compensate for the loss of D1:1, which resulted in disassembly of PSII at low temperature. These results imply translational rather than transcriptional regulation of psbA gene expression in Synechococcus 7942 at low temperature, and demonstrate the crucial role of the degree of membrane-lipid unsaturation in promotion of exchange of the D1 protein forms and thus the sustenance of PSII function at low temperature.

Membrane lipid unsaturation modulates processing of the photosystem II reaction-center protein D1 at low temperatures.

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 temperatures. 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 degrees C (growth temperature) to 18 degrees 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.

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.

Low unsaturation level of thylakoid membrane lipids limits turnover of the D1 protein of photosystem II at high irradiance.

Turnover of the D1 protein of photosystem II (PSII) was studied in mutants of Synechocystis sp. PCC 6803 defective in the desaturation of thylakoid membrane lipids. The lack of polyunsaturated fatty acids from membranes made PSII extremely susceptible to photoinhibition and caused a significant reduction in the D1 protein content of the thylakoid membranes at high irradiances. These results may be attributed to an impaired function in the protein synthesis machinery, most probably at the translational or posttranslational level.