CHLORORESPIRATION REVISITED - MITOCHONDRIAL-PLASTID INTERACTIONS IN CHLAMYDOMONAS

The changes of the redox state of plastoquinone and of the rate of bac k-reaction between the primary photoproducts of PS II centres have bee n studied following addition of myxothiazol to whole cells of C. reinh ardtii. Using appropriate mutant strains it is shown that these change s, namely a slow reduction of plastoquinone and a comparatively fast i ncrease in the rate of back-reaction, are a consequence of the inhibit ion of mitorespiration at the level of cytochrome be, complex. These n ew data are discussed in relation with our previous model of chlorores piration (Bennoun, P. (1982) Proc, Natl. Acad. Sci. USA 79, 4352-4356) . In contrast to these effects, a decrease in oxygen concentration aff ecting only slightly the rate of back-reaction induces a fast reductio n of plastoquinone, suggesting that the chloroplast oxidase has a low affinity for oxygen as compared to mitochondrial oxidases. The large v ariations in the rate of back-reaction that we could observe raise man y questions relevant to their origin and to their possible implication s, that are discussed in detail. If, as seems likely, these variations result from changes in the electrochemical gradient built up across t hylakoid membranes, then the existence of a new gradient generator sho uld be postulated.