The active site of photosynthetic water oxidation by Photosystem II (PSII) is a manganese-calcium cluster (Mn(4)CaO(5)). A postulated catalytic base is assumed to be crucial. CP43-Arg357, which is a candidate for the identity of this base, is a second-sphere ligand of the Mn(4)-Ca cluster and is located near a putative proton exit pathway, which begins with residue D1-D61. Transient absorption spectroscopy and time-resolved O(2) polarography reveal that in the D1-D61N mutant, the transfer of an electron from the Mn(4)CaO(5) cluster to Y(Z)(OX) and O(2) release during the final step of the catalytic cycle, the S(3)-S(0) transition, proceed simultaneously but are more dramatically decelerated than previously thought (t(1/2) of up to ~50 ms vs a t(1/2) of 1.5 ms in the wild type). Using a bare platinum electrode to record the flash-dependent yields of O(2) from mutant and wild-type PSII has allowed the observation of the kinetics of release of O(2) from extracted thylakoid membranes at various pH values and in the presence of deuterated water. In the mutant, it was possible to resolve a clear lag phase prior to the appearance of O(2), indicating formation of an intermediate before the onset of O(2) formation. The lag phase and the photochemical miss factor were more sensitive to isotope substitution in the mutant, indicating that proton efflux in the mutant proceeds via an alternative pathway. The results are discussed in comparison with earlier results obtained from the substitution of CP43-Arg357 with lysine and in regard to hypotheses concerning the nature of the final steps in photosynthetic water oxidation. These considerations led to the conclusion that proton expulsion during the initial phase of the S(3)-S(0) transition starts with the deprotonation of the primary catalytic base, probably CP43-Arg357, followed by efficient proton egress involving the carboxyl group of D1-D61 in a process that constitutes the lag phase immediately prior to O(2) formation chemistry.