ATP CONTROL ON STATE TRANSITIONS INVIVO IN CHLAMYDOMONAS-REINHARDTII

We have attempted to depress the ATP content in dark adapted cells of the unicellular green algae, C. reinhardtii, by inhibiting ATP synthesis coupled to mitochondrial electron flow. Whether we used uncouplers, ATP synthase inhibitors, inhibitors of mitochondrial electron transport or a mutant altered in mitochondrial cytochrome b, we observed a fluorescence quenching at room temperature associated with an increased reduction of the intersystem electron carriers of the photosynthetic apparatus. This fluorescence quenching reflected a genuine transition to state II, since: (i) it was associated with an increase in light-harvesting complex phosphorylation and (ii) it did not occur in mutants lacking the b6 f complex which are blocked in State I. The complete reversion to state I occurred only into experimental conditions which allowed both ATP synthesis and reoxidation of intersystem electron carriers. We conclude that the intracellular demand for ATP controls state transitions in vivo. The interplay between intracellular ATP concentrations and the redox state of the kinase activator at the thylakoid membrane level is discussed. © 1990.