Effects of Inactivating psbM and psbT on Photodamage and Assembly of Photosystem II in Synechocystis sp PCC 6803

PsbM and PsbT have been assigned to electron densities on both photosystem II (PSII) monomers at the PSII dimer interface in X-ray crystallographic structures from Thermosynechoccocus elongatus and T. vulcanus. Our results show that removal of either or both proteins from Synechocystis sp. PCC 6803 resulted in photoautotrophic strains but the Delta PsbM:Delta PsbT mutant did not form stable dimers. A CP43-less PSII monomer accumulated in both single mutants, although absence of PsbT destabilized PSII to a greater extent than removing PsbM. Additionally, Delta PsbT cells exhibited slowed electron transfer between the plastoquinone electron acceptors, Q(A) and Q(B); however, S-state cycling in both mutants was similar to wild type. Oxygen evolution in these mutants rapidly inactivated following exposure to high light where recovery required protein synthesis and could proceed in the dark in Delta PsbM cells but required light in Delta PsbT cells. Interestingly, the extent of recovery of oxygen-evolving activity was greatest in the Delta PsbM:Delta PsbT strain. We also found recovery required Psb27 in Delta PsbT cells although, under our conditions, the Delta Psb27 strain remained similar to wild type. In contrast, the Delta PsbM:Delta Psb27 mutant could not assemble PSII beyond a CP43-minus intermediate. Our results suggest essential roles for Psb27 in biogenesis in the Delta PsbM strain and for repair from photodamage in cells lacking PsbT.