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

In Photosystem II (PS II) the D2 and D1 proteins provide binding sites for the primary (Q_A) and secondary (Q_B) plastoquinone electron acceptors, respectively. A non-heme iron is located between Q_A and Q_B 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 Q_A^- to Q_B electron transfer, together with impaired exchange between the Q_B-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 Q_A and Q_B. 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 Q_B-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.