Mutations of basic arginine residue 334 in the D1 protein of Photosystem II lead to unusual S-2 state properties in Synechocystis sp PCC 6803

The C-terminus region of the D1 protein of Photosystem II (PS II) is situated on the lumenal side of the complex and is likely to be involved in the coordination of the active site Mn atoms of the water oxidation complex (WOC). The strictly conserved arginine at position 334 (D1-334) was targeted for site-directed mutagenesis to explore the hypothesis that it is involved in the PS II extrinsic protein binding, chloride binding, or proton transfer. Although it was found that D1-R334 probably not essential for these functions, mutations at this position were found to uniquely alter the kinetics of S-state cycling in general and the properties of the S-2 state in particular. Substitutions of a glutamate (D1-R334E) and a valine (D1-R334V) for D1-R334 lead to an unusually stable (t(1/2) > 30 min at room temp) S-2 state, but not S-3, as measured by double flash measurements on the bare platinum electrode. However, measurements of fluorescence decay in the presence of DCMU suggest the S-2 state is only modestly affected by the mutations. Possible reasons for these apparently contradictory results are discussed.