Differential effects of the D1/S264V mutation in Photosystem II with either PsbA1 or PsbA3 on Q _B , non-heme Iron, and the associated hydrogen-bond network

The role of the D1/S264 residue and the role of its environment in the proton-coupled electron transfer reaction on the acceptor side of Photosystem II were investigated. To this end, D1/S264V mutants were constructed in the thermophilic cyanobacterium Thermosynechococcus elongatus , with D1 being either PsbA1 or PsbA3. The PSII mutants were investigated using EPR spectroscopy, thermoluminescence, (time-resolved) absorption changes measurements, and oximetry. While the mutation had minor effects in PsbA1-PSII, the S264V mutation in PsbA3-PSII had significant consequences: i ) thermoluminescence data show inefficient electron transfer from QA ^- to QB; ii ) re-oxidation of QA ^- was slowed, by at least a factor of 10; iii ) the herbicides inhibit weakly O2 evolution; iv ) no Fe ²⁺ QB ^- EPR signal was detected in dark-adapted PSII; instead, v ) a large Fe ³⁺ signal was present with vi ) modified EPR properties; vii ) no QA ^- Fe ²⁺ QB ^- biradical signal was observed after illumination at 198 K following a flash illumination, confirming the inefficient formation of QB ^- ; viii ) either no proton uptake coupled to non-heme iron reduction occurred or with a very slow rate compared to PsbA3-PSII; ix ) changes were noted in the electrochromic response associated with QA ^- formation; and x ) increased production of singlet oxygen, both with and without herbicides. The S264V mutation in PsbA3-PSII leads to a significant decrease in the energy gap between the QA ^- QB and QAQB ^- states. The effects listed above are discussed regarding the differences between PsbA1-PSII and PsbA3-PSII as those related to the sulfoquinovosyldiacylglycerol, the water molecules and the H-bond network.