The Cytochrome bc1, Complex of Yeast Mitochondria

Abstract

Yeast [Saccharomyces carlsbergensis] cells were labeled with radioactive amino acids in the presence of cycloheximide and the cytochrome bc1 complex was isolated from them as described in a preceding paper. After analysis of this preparation by sodium dodecylsulfate polyacrylamide gel electrophoresis only 1 band, with an apparent Mr [relative MW] of 32,000, had incorporated radioactivity. The amount of label in the band was low, but could be increased approximately 5-fold by preincubating the cells in erythromycin before the labeling period. Cells were labeled in the presence of chloramphenicol, and the cytochrome bc1 complex was isolated by (NH4)2SO4 fractionation. Upon electrophoresis in the presence of sodium dodecylsulfate only 4 of the 6 bands that belong to the complex had incorporated radioactivity; no radioactivity was found in the bands with an Mr of 40,000 and 17,000. The same result was obtained after labeling in the presence of acriflavin. If the cytochrome bc1 complex was isolated by immunoprecipitation, all bands were found to have incorporated radioactivity in the presence of chloramphenicol. The amount of radioactivity in the Mr 32,000 band was clearly depressed. Of the 7 polypeptides of the cytochrome bc1 complex of yeast, only 1 is made on mitochondrial ribosomes. This polypeptide has an Mr of 32,000 and is probably associated with cytochrome b.