Light activates binding of membrane proteins to chloroplast RNAs in Chlamydomonas reinhardtii

Several membrane proteins were previously shown to bind to the 5′ leader of the chloroplast psbCmRNA in the unicellular eukaryotic alga Chlamydomonas reinhardtii. This study showed that these proteins have affinity for AU-rich RNAs, as determined by competition experiments. In addition, their binding activities are enhanced 13–15-fold by light, and a 46 kDa protein is activated within 1–10 min. This activation could be mediated by the modulation of ADP pools by the light-dependent reactions of photosynthesis and ATP synthase because (1) two inhibitors that block ATP synthesis also prevent this activation and (2) ADP inhibits the RNA-binding activity of this protein in vitro. An inhibitor of Photosystem II diminishes this induction, suggesting that reducing potential generated by the photosynthetic electron transport chain modulates this RNA-binding activity. The RNA-binding activities of two proteins (of 46 and 47 kDa) are inhibited by Mg-protoporphyrin IX methyl ester in vitrosuggesting they could be regulated by these intermediates in the chlorophyll biosynthetic pathway.