Studies on the cytochrome b 6 / f complex. I. Characterization of the complex subunits in $Chlamydomonas\ reinhardtii$
Abstract
We have analyzed the heme-associated peroxidase activity in thylakoid membranes from the. green algae
$Chlamydomonas\ reinhardtii$ after electrophoresis in the presence of sodium dodecyl sulfate. Besides
cytochrome $f$ and cytochrome $b_6$, we observed peroxidase activity in two other bands, of 34 and 11 kDa, of
unknown origin. Characterization of the $b_6/f$ complex subunits was undertaken by means of a comparison
of the polypeptide deficiencies in several $b_6/f$ mutants with the polypeptide content of preparations
enriched in $b_6/f$ complexes. We conclude that the $b_6/f$ complex consists of five subunits. Using
site-specific translation inhibitors, we show that cytochrome $f$, cytochrome $b_6$ and 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.
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