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 P680, in contrast to the earlier results from the analysis of the equivalent mutants constructed in Synechoeystis 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-His 198 and D2-His197 of photosystem II in Synechocystis PCC 6803: sites of primary charge separation and cation and triplet stabilization, Biochemistry 40 (2001) 9265-92811. We conclude that the nature of the axial ligand to PD1 is not an important determinant of the redox and spectroscopic properties Of P680 in T elongatus. (C) 2008 Elsevier B.V All rights reserved.