Dark deactivation and photoactivation of H2O-splitting activity were examined in a directed mutant (delta psbO) of Synechocystis sp. PCC6803 lacking the extrinsic manganese-stabilizing protein of the photosystem II (PSII) reaction center complex. Rapid (t1/2 = 10 min) losses of H2O-splitting activity were observed for delta psbO cells kept in the dark, but not for wild-type cells. The loss of H2O-splitting activity by delta psbO cells was suppressed by maintaining the cells under illumination and dark losses were rapidly (t1/2 < 1 min) reversed by light. Photoactivation kinetics of delta psbO and wild-type cells were compared following hydroxylamine extraction of PSII Mn. Photoactivation of delta psbO cells under continuous illumination occurs at an intrinsically faster rate (about 4-fold) than the wild-type. Virtually all of the increase in the rate of photoactivation can be accounted for by a corresponding 4-fold increase in the relative quantum yield of photoactivation as indicated by the yield of photoactivation as a function of flash number. The flash frequency dependence of photoactivation indicates a multi-quantum process in the mutant resembling the wild-type, but with significant increases in yields at all flash frequencies examined. The higher quantum yield of photoactivation in delta psbO cells occurs in the absence of large changes in the kinetics of the rate-limiting dark rearrangement. The results are consistent with increased accessibility (or affinity) and photooxidation of Mn2+ at one or both of the two binding sites involved in the initial stages of the photoactivation mechanism. In the context of previous results, it is proposed that MSP regulates the binding/photooxidation of the second Mn2+ of the photoligation sequence, but not the first.