PsbY is required for prevention of photodamage to photosystem II in a PsbM-lacking mutant of Synechocystis sp PCC 6803

The PsbM (3.9 kDa) and PsbY (4.2 kDa) proteins are membrane-spanning, single-helix, subunits associated with the chlorophyll-binding CP47 pre-complex of photosystem II (PSII). Removal of PsbM resulted in accumulation of PSII pre-assembly complexes and impaired electron transfer between the primary (Q(A)) and secondary (Q(B)) plastoquinone electron acceptors of PSII indicating that the Q(B)-binding site and bicarbonate binding to the non-heme iron were altered in this strain. Removal of PsbY alone had only a minor impact on PSII activity but deleting PsbY in the Delta PsbM background led to additional modification of the acceptor side resulting in Delta PsbM:Delta PsbY cells being susceptible to photodamage and this required protein synthesis for recovery. Addition of bicarbonate was able to compensate for the light-induced damage in Delta PsbM:Delta PsbY cells potentially re-occupying the modified bicarbonate-binding site in the Delta PsbM:Delta PsbY strain and complementation of Delta PsbM:Delta PsbY cells with the psbY gene restored the Delta PsbM phenotype.