Two functional sites of phosphatidylglycerol for regulation of reaction of plastoquinone QB in photosystem II

Functional roles of an anionic lipid phosphatidylglycerol (PG) were studied in pgsA-gene-inactivated and cdsA-gene-inactivated/phycobilisome-less mutant cells of a cyanobacterium Synechocystis sp. PCC 6803, which can grow only in PG-supplemented media. 1) A few days of PG depletion suppressed oxygen evolution of mutant cells supported by p-benzoquinone (BQ). The suppression was recovered slowly in a week after PG re-addition. Measurements of fluorescence yield indicated the enhanced sensitivity of Q(B) to the inactivation by BQ It is assumed that the loss of low-affinity PG (PG(L)) enhances the affinity for BQ that inactivates Q(B). 2) Oxygen evolution without BQ supported by the endogenous electron acceptors, was slowly suppressed due to the direct inactivation of Q(B) during 10 days of PG depletion, and was recovered rapidly within 10 h upon the PG re-addition. It is concluded that the loss of high-affinity PG (PG(H)) displaces Q(B) directly. 3) Electron microscopy images of PG-depleted cells showed the specific suppression of division of mutant cells, which had developed thylakoid membranes attaching phycobilisomes (PBS). 4) Although the PG-depletion for 14 days decreased the chlorophyll/PBS ratio to about 1/4, florescence spectra/lifetimes were not modified indicating the flexible energy transfer from PBS to different numbers of PSII. Longer PG-depletion enhanced allophycocyanin fluorescence at 683 nm with a long 1.2 ns lifetime indicating the suppression of energy transfer from PBS to PSII. 5) Action sites of PG(H), PG(L) and other PG molecules on PSII structure are discussed. (C) 2011 Elsevier B.V. All rights reserved.