Studies on the Photoactivation of the Water-Oxidizing Enzyme

Abstract

In weak yet optimal light intensity, complete photoactivation of the water-oxidizing enzyme in NH(2)OH-extracted wheat (Triticum aestivum, var Oasis) leaf segments could be obtained only after long dark preincubation. Photoactivation was not affected by ethylenediaminetetraacetate or inhibitors of photophosphorylation and protein synthesis, but was partially inhibited by a divalent cation ionophore. Complete photoactivation required ligation of approximately 4 Mn by the water oxidizing enzyme.WITHOUT DARK PREINCUBATION, PHOTOSYSTEM II (PSII) WAS SUSCEPTIBLE TO WEAK LIGHT PHOTOINHIBITION RESULTING IN: (a) 50% maximum decrease in photooxidation of artificial electron donors by PSII: (b) increased times for the variable fluorescence rise (with 3-(3,4-dichlorophenyl)-1,1-dimethyl urea): (c) abolishment of photoactivation: and (d) the imposition of sensitivity to inhibitors of photophosphorylation and 70S but not 80S protein synthesis on subsequent light-dependent recovery from photoinhibition and recovery of O(2) evolution. Decrease in susceptibility to photoinhibition and increase in rates of photoactivation resulting from dark preincubations proved closely correlated. Neither protein synthesis nor increases in abundances of thylakoid Mn(2+) and Ca(2+) were required for escape from photoinhibition. However, photoactivation of the wateroxidizing enzyme in NH(2)OH-extracted Chlamydomonas occurred in absence of dark preincubation and protein synthesis. Results are discussed in the context of disassembly/reassembly/resynthesis of specific PSII polypeptides.