Photophosphorylation Associated with Photosystem II

Abstract

Incubation of spinach chloroplast membranes for 90 min in the presence of 50 mM KCN and 100 .mu.M HgCl2 produces an inhibition of photosystem I activity which is stable to washing and to storage of the chloroplasts at -70.degree. C. Subsequent exposure of these preparations to NH2OH and ethylenediaminetetraacetic acid destroys O2 evolution and flow of electrons from water to oxidized p-phenylenediamine, but 2 types of phosphorylating cyclic electron flow was still observed. In the presence of 3-(3,4-dichlorophenyl)-1,1''-dimethylurea, phenazinemethosulfate catalyzes ATP synthesis at a rate 60% that observed in uninhibited chloroplasts. C-substituted p-phenylenediamines will also support low rates of photosystem I-catalyzed cyclic photophosphorylation, but p-phenylenediamine is completely inactive. When photosystem II is not inhibited, p-phenylenediamine will catalyze ATP synthesis at rates up to 90 .mu.mol/h .cntdot. mg chlorophyll. This reaction is unaffected by anaerobiosis, and an action spectrum for ATP synthesis shows a peak at 640 nm. These results are evidence for the existence of photosystem II-dependent cyclic photophosphorylation in these chloroplast preparations.