In situ effects of mutations of the extrinsic cytochrome c550 of photosystem II in Synechocystis sp PCC6803

The H2O oxidizing domain of the cyanobacterial photosystem II (PSII) complex contains a low potential, c-type cytochrome termed c(550) that is essential for the in vivo stability of the PSII complex. A mutant lacking cytochrome c(550) (DeltapsbV) in Synechocystis sp. PCC6803 has been further analyzed together with a construct in which the distal axial heme iron ligand, histidine 92, has been substituted with a methionine (C550-H92M). Heme staining of SDS-PAGE showed that the C550-H92M mutation did not disturb the accumulation and heme-binding properties of the cytochrome. In DeltapsbV cells, the number of charge separating PSII centers was estimated to be 56% of the wild type, but of the existing centers, 33% lacked photooxidizable Mn ions. C550-H92M did not discernibly affect the intrinsic PSII electron-transfer kinetics compared to the wild type nor did it exhibit a significant fraction of centers lacking photooxidizable Mn; however, the number of charge separating PSII centers in mutant cells was 69% of the wild type. C550-H92M lost photoautotrophic growth ability in the absence of Ca2+, but its growth was not affected by depletion of CI-, which differs from DeltapsbV. Taken together, the results suggest that in the absence of cytochrome c(550) electron transfer on the donor side is retarded perhaps at the level of Y-z to P680(divided by) transfer, the heme ligand. His92 is not absolutely required for assembly of functional PSII centers; however, replacement by methionine prevents normal accumulation of PSII centers in the thylakoid membranes and alters the Ca2+ requirement of PSIL The results are discussed in terms of current understanding of the Ca2+ site of PSII.