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Inhibition of HCO₃^– Binding to Photosystem II by Atrazine at a Low-Affinity Herbicide Binding Site ¹

Botany Department, University of California, Davis, California 95616

In maize chloroplasts, the ratio of HCO₃^– (anion) binding sites to high-affinity atrazine binding sites is unity. In the dark, atrazine noncompetitively inhibits the binding of half of the HCO₃^– to the photosystem II (PSII) complexes. The inhibition of binding saturates at 5 micromolar atrazine, little inhibition is seen at 0.5 micromolar atrazine, although the high-affinity herbicide binding sites are nearly filled at this concentration. This means that HCO₃^– and atrazine interact noncompetitively at a specific low-affinity herbicide binding site that exists on a portion of the PSII complexes. Light abolishes the inhibitory effects of atrazine on HCO₃^– binding. Based on the assumption that there is one high-affinity atrazine binding site per PSII complex, we conclude that there is also only one binding site for HCO₃^– with a dissociation constant near 80 micromolar. The location of the HCO₃^– binding site, and the low-affinity atrazine binding site, is not known.