Translational regulation of chloroplast gene expression during the light-dark cell cycle of Chlamydomonas: evidence for control by ATP/energy supply.

In Chlamydomonas reinhardtii growing synchronously under a light-dark cycle, the major chloroplast mRNAs are constitutively present but are translated only during the light period. We show that translation of these mRNAs can be induced during the normal dark period by light or by acetate and the induction is blocked by an inhibitor of ATP synthesis, CCCP. Moreover, ATP levels in synchronous cells were found to be 2-5-fold lower during the dark than in the light period; the administration of acetate or light at the mid-dark period increased the ATP level 2-3-fold. These results exclude cell-cycle mediated control and suggest that the regulation of chloroplast translation in light-dark grown Chlamydomonas is mediated, at least in part, by ATP levels.