Photophosphorylation Associated with Photosystem II: I. Photosystem II Cyclic Photophosphorylation Catalyzed by p-Phenylenediamine

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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.

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