The contribution of D2-Arg265 to the molecular architecture of the quinone-Fe-bicarbonate acceptor complex of photosystem II

Abstract

In Photosystem II (PS II) the D2 and D1 proteins provide binding sites for the primary (QA) and secondary (QB) plastoquinone electron acceptors, respectively. A non-heme iron is located between QA and QB that is coordinated by a bicarbonate ligand and two His residues from D1 (D1-His215 and D1-His272) and two His residues from D2 (D2-His214 and D2-His268). The symmetry of the quinone-Fe-acceptor complex extends to D1-Arg269, which has hydrogen bonds to D1-His272, and D2-Arg265 which has hydrogen bonds to D2-His268. We have examined the role of D2-Arg265 by creating the R265A and R265D mutants in the cyanobacterium Synechocystis sp. PCC 6803. Both mutants exhibited normal photoautotrophic growth, but showed a reduction in oxygen evolution in the presence of the PS II-specific electron acceptor 2,5-dimethyl-1,4-benzoquinone (DMBQ). Chlorophyll a fluorescence induction and decay kinetics were also inhibited in the presence of DMBQ and, in the presence of the native quinone, revealed slowed QA− to QB electron transfer, together with impaired exchange between the QB-binding site and the plastoquinone pool. Addition of formate further inhibited electron transfer, consistent with weakened bicarbonate binding in the mutants, and thermoluminescence measurements revealed a decreased redox gap between QA and QB. Additionally, both mutants displayed heightened sensitivity to high light. These findings demonstrate that D2-Arg265 is important for stability of the acceptor side, bicarbonate-dependent electron transfer, and an optimal QB-binding site. All of our results are also consistent with the architecture of the quinone-Fe-bicarbonate complex supporting photoprotection and regulatory roles that are unique to oxygenic photosynthesis.