Studies on the cytochrome b6/fcomplex. I. Characterization of the complex subunits in Chlamydomonas reinhardtii

We have analyzed the heme-associated peroxidase activity in thylakoid membranes from the green algae Chlamydomonas reinhardtiiafter electrophoresis in the presence of sodium dodecyl sulfate. Besides cytochrome fand cytochrome b6, we observed peroxidase activity in two other bands, of 34 and 11 kDa, of unknown origin. Characterization of the b6/fcomplex subunits was undertaken by means of a comparison of the polypeptide deficiencies in several b6/fmutants with the polypeptide content of preparations enriched in b6/fcomplexes. We conclude that the b6/fcomplex consists of five subunits. Using site-specific translation inhibitors, we show that cytochrome f, cytochrome b6and subunit IV are of chloroplast origin, whereas the Rieske protein and probably subunit V are translated on cytoplasmic ribosomes. A model of assembly of the complex is proposed: a cytochrome moiety, comprising the subunits of chloroplast origin, is assembled in the thylakoid membranes prior to the insertion and assembly of the subunits encoded in the nuclear genome.