Aspartate 170 of the Photosystem II Reaction Center Polypeptide D1 is Involved in the Assembly of the Oxygen-evolving Manganese Cluster

Eleven site-directed mutations were constructed at aspartate 170 of the D1 polypeptide of the photosystem II (PSII) reaction center of the cyanobacterium Synechocystis sp. PCC 6803. The light-saturated rates of O2 evolution (VO2) measured in whole cells range from close to that of wild-type for Asp170Glu to zero for Asp170Ser and Ala. Those mutant strains that are best able to evolve O2 are also those that show the lowest Km in PSII core complexes for the oxidation of Mn2+ by oxidized Tyr161, the normal oxidant of the Mn cluster responsible for O2 evolution. To a first approximation, the lower the pKa of the residue at position 170, the higher the VO2 and the lower the Km. D1-Asp170 appears to participate in the early steps associated with the assembly of the Mn cluster. It is also the first reported example of an amino acid residue critical to the function and assembly of the oxygen-evolving complex.