Turnover of MCA1, the stabilization factor of petA mRNA, is coupled to the rate of translation of petA through its interaction with newly synthesised cytochrome f

A. Boulouis, C. Raynaud, S. Bujaldon, A. Aznar, F.A. Wollman, and Y. Choquet

Laboratoire de Physiologie Membranaire et Moléculaire du Chloroplaste, UMR7141 CNRS-UPMC Paris-6, IBPC, 13 rue Pierre et Marie Curie 75005 Paris

In Chlamydomonas chloroplasts, expression of cytochrome f, a major subunit of the cytochrome b6f complex encoded by the chloroplast petA gene, depends on two nucleus encoded factors, MCA1 and TCA1, respectively required for the accumulation and the translation of the petA mRNA. These two factors share partially overlapping functions, with MCA1 acting as a translational enhancer for the TCA1-mediated synthesis of cytochrome f. We provide extensive evidence for a molecular interaction between these two factors that form high molecular weight complexes that also contain the petA mRNA. We observed that the half-life of MCA1 is longer when cytochrome f translation is prevented, either using translational inhibitors or site-directed mutagenesis. However, petA mRNA translation does not directly control the half-life of MCA1. Rather, an interaction between MCA1 and newly synthesized cytochrome f is responsible for the proteolytic disposal of the nuclear factor. Strikingly, this interaction is mediated by the very same residues at the C-terminus of cytochrome f that are involved in the CES behaviour of the protein. These cytochrome f residues form a repressor motif contributing, with the 5' UTR of petA mRNA, to a negative feed-back mechanism, the CES process, that down-regulates translation of cytochrome f when its assembly within cytochrome b6f complexes is compromised. Based on these new findings, we present a refined model for the CES process that involves the MCA1 protein.

e-mail address of presenting author: choquet@ibpc.fr

web site: http://www.ibpc.fr/UMR7141/