Differential effects of the D1/S264V mutation in photosystem II with either PsbA1 or PsbA3 on QB, 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 Q_A⁻ to Q_B; ii) re-oxidation of Q_A⁻ was slowed, by at least a factor of 10; iii) the herbicides inhibit weakly O₂ evolution; iv) no Fe²⁺Q_B⁻ EPR signal was detected in dark-adapted PSII; instead, v) a large Fe³⁺ signal was present with vi) modified EPR properties; vii) no Q_A⁻Fe²⁺Q_B⁻ biradical signal was observed after illumination at 198 K following a flash illumination, confirming the inefficient formation of Q_B⁻; 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 Q_A⁻ 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 Q_A⁻Q_B and Q_AQ_B⁻ 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.