Assembly-controlled regulation of chloroplast gene translation

Studies of the biogenesis of the photosynthetic protein complexes in the un icellular green alga Chlamydomonas reinhardtii have pointed to the importan ce of the concerted expression of nuclear and chloroplast genomes. The accu mulation of chloroplast- and nuclear-encoded subunits is concerted, most of ten as a result of the rapid proteolytic disposal of unassembled subunits, but the rate of synthesis of some chloroplast-encoded subunits from photosy nthetic protein complexes, designed as CES proteins (Controlled by Epistast y of Synthesis), is regulated by the availability of their assembly partner s from the same complex. Cytochrome f, a major subunit of the cytochrome b( 6)f complex is a model protein for the study of the CES process. In the abs ence of subunit IV, another subunit of the cytochrome b(6)f complex, its sy nthesis is decreased by 90%. This results from a negative autoregulation of cytochrome f translation initiation, mediated by a regulatory motif carrie d by the C-terminal domain of the unassembled protein [Choquet, Stern, Wost rikoff, Kuras, Girard-Bascou and Wollman (1998) Proc. Natl. Acad. Sci. U.S. A. 95, 4380-4385]. Using site-directed mutagenesis, we have characterized t his regulatory motif. We discuss the possible implications regarding the me chanism of the CES process for cytochrome f expression. We have studied the possible generalization of this mechanism. to other CES proteins.