Spectroscopic and functional characterizations of cyanobacterium Synechocystis PCC 6803 mutants on and near the heme axial ligand of cytochrome b559 in photosystem II

Abstract

The functional role of cytochrome (cyt) b559 in photosystem II (PSII) was investigated in H22Kα and Y18Sα cyt b559 mutants of the cyanobacterium Synechocystis sp. PCC6803. H22Kα and Y18Sα cyt b559 mutant carries one amino acid substitution on and near one of heme axial ligands of cyt b559 in PSII, respectively. Both mutants grew photoautotrophically, assembled stable PSII, and exhibited the normal period-four oscillation in oxygen yield. However, both mutants showed several distinct chlorophyll a fluorescence properties and were more susceptible to photoinhibition than wild type. EPR results indicated the displacement of one of the two axial ligands to the heme of cyt b559 in H22Kα mutant reaction centers, at least in isolated reaction centers. The maximum absorption of cyt b559 in Y18Sα mutant PSII core complexes was shifted to 561 nm. Y18Sα and H22Kα mutant PSII core complexes contained predominately the low potential form of cyt b559. The findings lend support to the concept that the redox properties of cyt b559 are strongly influenced by the hydrophobicity and ligation environment of the heme. When the cyt b559 mutations placed in a D1-D170A genetic background that prevents assembly of the manganese cluster, accumulation of PSII is almost completely abolished. Overall, our data support a functional role of cyt b559 in protection of PSII under photoinhibition conditions in vivo. © 2010 by The American Society for Biochemistry and Molecular Biology, Inc.