Requirement of phosphatidylglycerol for photosynthetic function in thylakoid membranes

To investigate the role of phosphatidylglycerol (PC) in photosynthesis, we constructed a mutant defective in the CDP-diacyiglycerol synthase gene from a cyanobacterium. Synechocystis sp. PCC6803. The mutant, designated as SNC 1, required PG supplementation for growth. Growth was repressed in PG-free medium concomitantly with the decrease in cellular content of PC. These res ults indicate that PG is essential, and that SNC1 is defective in PG synthe sis. Decrease in PC content was accompanied by a reduction in the cellular content of chlorophyll. but with little effect on the contents of phycobili some pigments, which showed that levels of chlorophyll-protein complexes de creased without alteration of those of phycobilisomes. Regardless of the de crease in the PG content. CO2-dependent photosynthesis by SNC1 was similar to that by the wild type on a chlorophyll basis, but consequently became lo wer on a cell basis. Simultaneously, the ratio of oxygen evolution of photo system II (PSII) measured with p-benzoquinone to that of CO2-dependent phot osynthesis, which ranged between 1.3 and 1.7 in the wild type. However. it was decreased in SNC1 from 1.3 to 0.4 during the early growth phase where c hlorophyll content and CO2-dependent photosynthesis were little affected, a nd then finally to 0.1. suggesting that PSII first lost its ability to redu ce p-benzoquinone and then decreased in its level and actual activity. Thes e results indicate that PG contributes to the accumulation of chlorophyll-p rotein complexes in thylakoid membranes, and also to normal functioning of PSII.