1995 Volume 59 Issue 5 Pages 796-800
The binding constant (K) and number of binding sites (N) of atrazine to isolated photosystem (PS) II membranes were measured with an apparent correlation between N and the activity of oxygen evolution. Upon the addition of an electron acceptor, N became equal to the total number of the population of PS II reaction centers irrespective of having oxygen-evolving activity, about 4 mmol per mole of chlorophyll, with a concomitant decline of K from 1.32 (±0. 34)x107M-1 to 4.09 (±0.40)x106M-1. NH2OH and NaCl treatments, which inactivate oxygen evolution, affected neither the binding to PS II membranes of the extrinsic 33-kDa protein or of atrazine. The atrazine binding sites that are latent in CaCl2-treated PS II membranes was partially restored by the reconstitution of the membranes with isolated extrinsic 33-kDa protein. An oxidizing system involving the 33-kDa protein may provide a suitable structure of PS II reaction center complex for atrazine binding. The level of inhibition of oxygen-evolving activity by atrazine under the saturating intensity of light parallels the fraction of the photosystem (PS) II reaction center with the quinone-binding site blocked by atrazine. In contrast, under a rate-limiting intensity of light, percents of remaining oxygen-evolving activity after the addition of atrazine correlated with the 1.33th power of the fraction of atrazine-free binding sites. Inhibition of PS II complexes more than one that bound with atrazine suggests a cooperation between PS II complexes to evolve oxygen under weak light intensity.
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