THE MANGANESE-STABILIZING PROTEIN OF PHOTOSYSTEM-II MODIFIES THE IN-VIVO DEACTIVATION AND PHOTOACTIVATION KINETICS OF THE H2O OXIDATION COMPLEX IN SYNECHOCYSTIS SP PCC6803

Dark deactivation and photoactivation of H2O-splitting activity were e xamined in a directed mutant (Delta psbO) of Synechocystis sp. PCC6803 lacking the extrinsic manganese-stabilizing protein of the photosyste m Il (PSII) reaction center complex. Rapid (t(1/2) 10 min) losses of H 2O-splitting activity were observed for Delta psbO cells kept in the d ark, but not for wild-type cells. The loss of H2O-splitting activity b y Delta psbO cells was suppressed by maintaining the cells under illum ination and dark losses were rapidly (t1/2 < 1 min) reversed by light. Photoactivation kinetics of Delta psbO and wild-type cells were compa red following hydroxylamine extraction of PSII Mn. Photoactivation of Delta psbO cells under continuous illumination occurs at an intrinsica lly faster rate (about LF-fold) than the wild-type. Virtually all of t he increase in the rate of photoactivation can be accounted for by a c orresponding Lt-fold increase in the relative quantum yield of photoac tivation as indicated by the yield of photoactivation as a function of flash number. The flash frequency dependence of photoactivation indic ates a multi-quantum process in the mutant resembling the wild-type, b ut with significant increases in yields at all flash frequencies exami ned. The higher quantum yield of photoactivation in Delta psbO cells o ccurs in the absence of large changes in the kinetics of the rate-limi ting dark rearrangement. The results are consistent with increased acc essibility (or affinity) and photooxidation of Mn2+ at one or both of the two binding sites involved in the initial stages of the photoactiv ation mechanism. Tn the context of previous results, it is proposed th at MSP regulates the binding/photooxidation of the second Mn2+ of the photoligation sequence, but not the first.