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Bicarbonate-Reversible and Irreversible Inhibition of Photosystem II by Monovalent Anions ¹

Département de Biologie, Service de Radioagronomie, Centre d'Etudes Nucléaires de Cadarache, B.P. No. 1, F-13115 Saint-Paul-lez-Durance, France, Botany Department, University of California, Davis, California 95616

We tested a number of inhibitory monovalent anions for their primary site of action on photosystem II(PSII) in chloroplasts. We find that the inhibitory effects of F^–, HCO₂^–, NO₂^–, NO₃^–, and CH₃CO₂^– are all reversed by addition of a high concentration of HCO₃^–. This class of anions competitively inhibits H¹⁴CO₃^– binding to PSII. All of those anions tested reduced H¹⁴CO₃^– binding more in the light than in the dark. We conclude that the primary inhibitory site of action of a number of monovalent anions is at the HCO₃^– binding site(s) on the PSII complex. The carbonic anhydrase inhibitor gold cyanide, and also azide, inhibit PSII but at a site other than the HCO₃^– binding site. We suggest that the unique ability of HCO₃^– to reverse the effects of inhibitory anions reflects its singular ability to act as a proton donor/acceptor at the anion binding site. A similar role has been proposed for non-substrate-bound HCO₃^– on carbonic anhydrase by Yeagle et al. (1975 Proc Natl Acad Sci USA 72: 454-458).

¹ Supported by National Science Foundation Grant PCM 80 04075.