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Site-specific Inhibition of Photophosphorylation in Isolated Spinach Chloroplasts by Mercuric Chloride ¹

J. Michael Gould^b and Donald R. Ort^b

^a Department of Biological Sciences, University of Cincinnati, Cincinnati, Ohio 45221, Department of Botany and Plant Pathology, Michigan State University, East Lansing, Michigan 48824

Photophosphorylation associated with noncyclic electron transport in isolated spinach (Spinacia oleracea) chloroplasts is inhibited to approximately 50% by low concentrations of HgCl₂ (less than 1 µmole Hg²⁺/mg chlorophyll) when the electron transport pathway includes both sites of energy coupling. Reactions involving only a part of the electron transport system can give a functional isolation of at least two sites coupled to phosphorylation. Only one of these sites, located between the oxidation of plastoquinone and the reduction of cytochrome f, is sensitive to mercuric chloride. The energy conservation site located before plastoquinone and close to photosystem II is unaffected by HgCl₂ concentrations up to 10-fold those required to inhibit phosphorylation by the coupling site after plastoquinone. This site-specific inhibition may reflect a mechanistic difference in the mode of energy coupling at the two coupling sites or a variable accessibility of HgCl₂ to these sites.

Concentrations of HgCl₂, which inhibit steady state phosphorylation, do not inhibit dark phosphorylation after illumination (X_E), suggesting that HgCl₂ affects a step in the coupling mechanism prior to the terminal step of ATP formation.