Assembly-controlled regulation of chloroplast gene translation

Studies of the biogenesis of the photosynthetic protein complexes in the unicellular green alga Chlamydomonas reinhardtii have pointed to the importance of the concerted expression of nuclear and chloroplast genomes. The accumulation of chloroplast- and nuclear-encoded subunits is concerted, most often as a result of the rapid proteolytic disposal of unassembled subunits, but the rate of synthesis of some chloroplast-encoded subunits from photosynthetic protein complexes, designed as CES proteins (Controlled by Epistasy of Synthesis), is regulated by the availability of their assembly partners from the same complex. Cytochrome f, a major subunit of the cytochrome b₆f complex is a model protein for the study of the CES process. In the absence of subunit IV, another subunit of the cytochrome b₆f complex, its synthesis is decreased by 90%. This results from a negative autoregulation of cytochrome f translation initiation, mediated by a regulatory motif carried by the C-terminal domain of the unassembled protein [Choquet, Stern, Wostrikoff, Kuras, Girard-Bascou and Wollman (1998) Proc. Natl. Acad. Sci. U.S.A. 95, 4380–4385]. Using site-directed mutagenesis, we have characterized this regulatory motif. We discuss the possible implications regarding the mechanism of the CES process for cytochrome f expression. We have studied the possible generalization of this mechanism to other CES proteins.