Phosphatidylglycerol an essential structural and functional constituent of photosynthetic membrane

Abstract

The role of phosphatidylglycerol (PG) was studied in a Synechocystis sp. PCC6803 mutant to synthesize PG. The PG content in the membranes was controlled by the externally added PG. With the loss of PG content we observed a decrease in the photosynthetic oxygen evolving activity. It was accompanied with a significant decrease in the chlorophyll content of the cells. The change in photosynthetic activity was caused by the inhibition of electron transfer on the donor side between QA and QB plastoquinones. It was measured by oxygen evolution, induction of chlorophyll fluorescence, change in fast fluorescence yield detected by double flash technique and thermoluminescence. In the absence of PG, oxygen evolution was inhibited by the addition of artificial quinones. The altered fluorescence induction suggested the inactivation of the QB function in the PG-depleted cells. A fast change in fluorescence yield was indicated by slow (20 ms) reduction of P680+, and a very slow re-oxidation (1s) of QA. Thermoluminescence measurement also confirmed the inhibition of electron transfer from QA- to QB in the PG-depleted cells and suggested the accumulation of Yz+ and His+ in addition to S2 state. No significant change of redox level of QA and S2 state was detected. These findings demonstrate that loss of PG leads to the (1) inactivation of the QB function, (2) inhibition of the turnover of water splitting reaction and (3) modification of energy transfer from antenna to reaction center complex. Thus PG molecules seem to be indispensable structural components of the active PS II reaction center.