To analyze the role of phosphatidylglycerol (PG) in photosynthetic membranes of cyanobacteria we used two mutants of Synechocystis sp. PCC6803: the PAL mutant which has no phycobilisomes and shows a high PSII/PSI ratio, and a mutant derived from it by inactivating its cdsA gene encoding cytidine 5'-diphosphate diacylglycerol synthase, a key enzyme in PG synthesis. In a medium supplemented with PG the PAL/DeltacdsA mutant cells grew photoautotrophically. Depletion of PG in the medium resulted (a) in an arrest of cell growth and division, (b) in a slowdown of electron transfer from the acceptor Q(A) to Q(B) in PSII and (c) in a modification of chlorophyll fluorescence curve. The depletion of PG affected neither the redox levels of Q(A) nor the S(2) state of the oxygen-evolving manganese complex, as indicated by thermoluminescence studies. Two-dimensional PAGE showed that in the absence of PG (a) the PSII dimer was decomposed into monomers, and (b) the CP43 protein was detached from a major part of the PSII core complex. [(35)S]-methionine labeling confirmed that PG depletion did not block de novo synthesis of the PSII proteins. We conclude that PG is required for the binding of CP43 within the PSII core complex.