The PsbY protein is not essential for oxygenic photosynthesis in the cyanobacterium Synechocystis sp PCC 6803

A tetra-manganese cluster in the photosystem II (PSII) pigment-protein comp lex plays a critical role in the photosynthetic oxygen evolution process. P sbY, a small membrane-spanning polypeptide, has recently been suggested to provide a ligand for manganese in PSII (A.E. Gau, H.H. Thole, A. Sokolenko, L. Altschmied, R.G. Herrmann, E.K. Pistorius [1998] Mol Gen Genet 260: 56- 68). We have constructed a mutant strain of the cyanobacterium Synechocysti s sp. PCC 6803 with an inactivated psbY gene (sml0007). Southern-blot and p olymerase chain reaction analysis showed that the mutant had completely seg regated. However, the Delta psbY mutant cells grew normally under photoauto trophic conditions. Moreover, growth of the wild-type and mutant cells were similar under highlight photoinhibition conditions, as well as in media wi thout any added manganese, calcium, or chloride, three required inorganic c ofactors for the oxygen-evolving complex of PSII. Analysis of steady-state and flash-induced oxygen evolution, fluorescence induction, and decay kinet ics, and thermoluminescence profiles demonstrated that the Delta psbY mutan t cells have normal photosynthetic activities. We conclude that the PsbY pr otein in Synechocystis 6803 is not essential for oxygenic photosynthesis an d does not provide an important binding site for manganese in the oxygen-ev olving complex of PSII.