Phosphatidylglycerol (PG), a ubiquitous constituent of thylakoid membranes of chloroplasts and cyanobacteria, is demonstrated to be essential for the functionality of plastoquinone electron acceptor QB in the photosystem II reaction center of oxygenic photosynthesis. Growth of the pgsA mutant cells of Synechocystis sp. PCC6803 that are defective in phosphatidylglycerolphosphate synthase and are incapable of synthesizing PG, in a medium without PG, resulted in a 90% decrease in PG content and a 50% loss of photosynthetic oxygen-evolving activity as reported [Hagio, M., Gombos, Z., Várkonyi, Z. Masamoto, K., Sato, N., Tsuzuki, M., and Wada, H. (2000) Plant Physiol. 124, 795-804]. We have studied each step of the electron transport in photosystem II of the pgsA mutant to clarify the functional site of PG. Accumulation of QA- was indicated by the fast rise of chlorophyll fluorescence yield under continuous and flash illumination. Oxidation of QA - by QB plastoquinone was shown to become slow, and QA- reoxidation required a few seconds when measured by double flash fluorescence measurements. Thermoluminescence measurements further indicated the accumulation of the S2QA - state but not of the S2QB - state following the PG deprivation. These results suggest that the function of QB plastoquinone was inactivated by the PG deprivation. We assume that PG is an indispensable component of the photosystem II reaction center complex to maintain the structural integrity of the QB-binding site. These findings provide the first clear identification of a specific functional site of PG in the photosynthetic reaction center.
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