Role of Arg180 of the D2 protein in photosystem II structure and function

On the basis of sequence comparison with the M subunit of the reaction center of purple bacteria, no residues in photosystem II can be clearly identified that may be predicted to correspond to the His residue that binds one of the accessory bacteriochlorophylls in the purple bacterial reaction center. However, the Arg180 residue of the D2 protein is close to where this residue is predicted to be and could conceivably serve as a chlorophyll ligand. To analyze the function of Arg180, it was changed to nine different amino acids in the cyanobacterium Synechocystis sp. PCC 6803. Except for the Arg180→Gln (R180Q) mutant, the resulting strains were no longer photoautotrophic. The properties of photosystem II upon mutation of Arg180 were probed in strains from which photosystem I had been deleted genetically. Mutations at the Arg180 residue affected oxygen evolution capacity and the amount of photosystem II that was present in thylakoids. Surprisingly, in the Arg180 mutants, EPR signals that may originate from the oxidized redox-active Tyr160 of the D2 protein (Y(D)(ox)) were small and generally did not resemble the usual signal II(s), signifying an effect of the Arg180 mutations on the environment surrounding Tyr160. In addition, in most mutants, the charge recombination kinetics between the primary electron-accepting quinone in photosystem II (Q(A)^-) and oxidized species on the donor side were faster upon introducing mutations at Arg180 suggesting an increased steady-state concentration of P680⁺ in the mutants. However, Arg180 mutations also affected Q(A)^- oxidation by the secondary electron-accepting quinone (Q(B)). HPLC analysis showed that, in the Arg180 mutants that were assayed, the pheophytin/chlorophyll ratio of photosystem II had not changed, indicating that the mutations did not lead to a pheophytinization of one of the chlorophyll molecules. Even though the results,presented do not provide positive evidence that Arg180 of the D2 protein corresponds in function to the ligand to the central Mg in an accessory bacteriochlorophyll in reaction centers of purple bacteria, it is clear that changes in Arg180 greatly affect Tyr160 and P680. Various scenarios are discussed that are compatible with the data presented, and include an apparently close interaction between Arg180, His189, and Tyr160, and the possibility of the involvement of multiple chlorophylls to together form P680.