PHOTOACTIVATION OF THE WATER-OXIDIZING COMPLEX OF PHOTOSYSTEM-II CORE COMPLEX DEPLETED OF FUNCTIONAL MN

Light-induced reassembly (photoactivation) of the water-oxidizing complex was studied using the PS Il core complex (TSF2a), which is depleted of functional Mn, PS Il extrinsic proteins and light-harvesting chlorophyll proteins. O2 evolution activity of 300-mu-mol O2/Mg Chl per h was restored by incubation of TSF2a with a photoactivation medium containing 2 mM MnCl2, 10 mM CaCl2 and the 33/23/17 kDa proteins, under conditions of weak light illumination. The optimum pH for the photoactivation was pH 5.3, that is one pH unit more acidic than for Triton-treated PS II membranes. PS II donor activity and the affinity of Mn2+ for the high-affinity Mn2+-binding sites were lower at pH 5.5 than at pH 6.5. The dependence of photoactivation yield using repetitive flashes on the dark intervals between flashes indicated that both the rate constant of generation and rate constant of decay of the unstable intermediate state formed during a photoactivation process drastically decreases with decreases in the pH. These results suggest that lowering the pH leads to destabilization of the intermediate but increases the quantum efficiency of generation of the intermediate.