Roles of acidic lipids in photosynthesis

Abstract

We produced mutants of Synechocystis sp. PCC6803 defective in phosphatidylglycerol (PG) or sulfoquinovosyl diacylglycerol (SQDG) synthesis to investigate the roles of these acidic lipids in photosynthesis. The mutant as to PG, CDP-diacylglycerol synthase gene (cdsA) of which was disrupted by homologous recombination, required PG supplementation for its growth. The cdsA mutant, shifted from PG-supplemented to PG-free medium, showed a decrease in cellular content of PG with the repressed growth. The decrease in PG content was accompanied with a reduction in cellular content of Chl, which was accounted for largely by a reduction of content of PSI complex. Simultaneously, p-benzoquinone-dependent oxygen evolution, higher than photosynthesis before the shift, became lower than photosynthesis, indicating some damage in PSII. On the other hand, the mutant as to SQDG, sqdB gene of which was disrupted, required SQDG supplementation for its growth. The sqdB mutant showed a decrease in cellular content of SQDG with the repressed growth after the shift from SQDG-supplemented to SQDG-free medium. Simultaneously, the sqdB mutant, in contrast to cdsA mutant, showed no decrease in Chl content. However, with the decrease in SQDG content, sensitivity of photosynthesis to DCMU was elevated, indicating that PSII was impaired. These results indicate that, for Synechocystis sp. PCC6803, PG and SQDG are respectively essential and responsible for normal functioning of PSII. However, accumulation of Chl-protein complexes in the thylakoid membranes are contributed by PG, but not by SQDG.