PsbY is required for prevention of photodamage to photosystem II in a PsbM-lacking mutant of <Emphasis Type="Italic">Synechocystis</Emphasis> 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 ΔPsbM background led to additional modification of the acceptor side resulting in ΔPsbM:ΔPsbY 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 ΔPsbM:ΔPsbY cells potentially re-occupying the modified bicarbonate-binding site in the ΔPsbM:ΔPsbY strain and complementation of ΔPsbM:ΔPsbY cells with the psbY gene restored the ΔPsbM phenotype.