Influence of Histidine-198 of the D1 subunit on the properties of the primary electron donor, P 680, of photosystem II in Thermosynechococcus elongatus

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The influence of the histidine axial ligand to the P D1 chlorophyll of photosystem II on the redox potential and spectroscopic properties of the primary electron donor, P 680, was investigated in mutant oxygen-evolving photosystem II (PSII) complexes purified from the thermophilic cyanobacterium Thermosynechococcus elongatus. To achieve this aim, a mutagenesis system was developed in which the psbA 1 and psbA 2 genes encoding D1 were deleted from a His-tagged CP43 strain (to generate strain WT ⁎) and mutations D1-H198A and D1-H198Q were introduced into the remaining psbA 3 gene. The O 2-evolving activity of His-tagged PSII isolated from WT ⁎ was found to be significantly higher than that measured from His-tagged PSII isolated from WT in which psbA 1 is expected to be the dominantly expressed form. PSII purified from both the D1-H198A and D1-H198Q mutants exhibited oxygen-evolving activity as high as that from WT ⁎. Surprisingly, a variety of kinetic and spectroscopic measurements revealed that the D1-H198A and D1-H198Q mutations had little effect on the redox and spectroscopic properties of P 680, in contrast to the earlier results from the analysis of the equivalent mutants constructed in Synechocystis sp. PCC 6803 [B.A. Diner, E. Schlodder, P.J. Nixon, W.J. Coleman, F. Rappaport, J. Lavergne, W.F. Vermaas, D.A. Chisholm, Site-directed mutations at D1-His198 and D2-His197 of photosystem II in Synechocystis PCC 6803: sites of primary charge separation and cation and triplet stabilization, Biochemistry 40 (2001) 9265–9281]. We conclude that the nature of the axial ligand to P D1 is not an important determinant of the redox and spectroscopic properties of P 680 in T. elongatus.