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Photophosphorylation Associated with Photosystem II

I. Photosystem II Cyclic Photophosphorylation Catalyzed by p-Phenylenediamine

¹ Department of Cell and Molecular Biology, Division of Biological Sciences, The University of Michigan, Ann Arbor, Michigan 48109

Incubation of spinach chloroplast membranes for 90 minutes in the presence of 50 mM KCN and 100 µM HgCl₂ produces an inhibition of photosystem I activity which is stable to washing and to storage of the chloroplasts at –70 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 two types of phosphorylating cyclic electron flow can still be 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 µmol/hr·mg chlorophyll. This reaction is unaffected by anaerobiosis, and an action spectrum for ATP synthesis shows a peak at 640 nm. These results are interpreted as evidence for the existence of photosystem II-dependent cyclic photophosphorylation in these chloroplast preparations.