ABSTRACIWe 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 P, HC02-, NO2-, reversed by addition of a high concentration of HC03-. This class of anions competitively inhibits H`4CO3-binding to PSII. All of those anions tested reduced H4CO3-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 HC03-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 HC03-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 MATERIALS AND METHODS Maize (Zea mays L.) plants were grown in a greenhouse, and chloroplast thylakoids were isolated from them as previously described (14). Osmotically broken, washed grana were used.The procedure for labeling membranes with H14CO3-has been described in detail (18). The only modifications to this procedure were that here the labeling was done at 20°C and the solution used to wash the chloroplasts on the Whatman GF/A filters contained 0.1 M Na-phosphate (pH 7.6) and 10 mM NaCl. 02 evolution in continuous saturating light was monitored with a Rank Brothers, Clark-type electrode. The light source was a Sylvania 1 50-w projector lamp focused by a Prado slide projector.The light passed through a Coming 3-67 orange filter.
RESULTS
Inhibition of ElectronTransport by Anions. We tested the effects ofvarious anions on electron transport through PSII under high intensity continuous light. The results are shown in Table I. F-, HCO2 , N02, Ac, NO3-, N3 , and the carbonic anhydrase inhibitor gold cyanide (Au (CN)2 ), all inhibit electron transport. It appears that smaller anions are more inhibitory than larger ones. Formate, for example, is more inhibitory than acetate while 20 mm NO2-is as effective as 100 mM NO3-. F-, the smallest anion, is relatively weak; however, it is hydrated in solution and thus has a larger effective radius. The inhibitory effects of F-, HCO2-, N02 , Ac, and NO3-are all reversed to a great extent by the simultaneous presence of 5 mm HCO3-. In contrast, the inhibition of electron transport by Au (CN)