Abstract
The changes of the redox state of plastoquinone and of the rate of back-reaction between the primary photoproducts of PS II centres have been studied following addition of myxothiazol to whole cells of C. reinhardtii. Using appropriate mutant strains it is shown that these changes, namely a slow reduction of plastoquinone and a comparatively fast increase in the rate of back-reaction, are a consequence of the inhibition of mitorespiration at the level of cytochrome bc 1 complex. These new data are discussed in relation with our previous model of chlororespiration (Bennoun, P. (1982) Proc. Natl. Acad. Sci. USA 79, 4352–4356). In contrast to these effects, a decrease in oxygen concentration affecting only slightly the rate of back-reaction induces a fast reduction of plastoquinone, suggesting that the chloroplast oxidase has a low affinity for oxygen as compared to mitochondrial oxidases. The large variations in the rate of back-reaction that we could observe raise many questions relevant to their origin and to their possible implications, that are discussed in detail. If, as seems likely, these variations result from changes in the electrochemical gradient built up across thylakoid membranes, then the existence of a new gradient generator should be postulated.
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