The cytochrome bc) complex of yeast mitochondria. Site of translation of the polypeptides in vivo.

1. Yeast cells were labelled with radioactive amino acids in the presence of cycloheximide and the cytochrome bc1 complex was isolated from them as described in the preceding paper (Katan, M.B.., Pool, L. & Groot, G.S.P. (1976)Eur. J. Biochem, 65, 95-105). After analysis of this preparation by sodium dodecylsulphate polyacrylamide gel electrophoresis only one band, with an apparent Mr of 32000, was found to have 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 labelling period. 2. Cells were labelled in the presence of chloramphenicol and the cytochrome bc1 complex was isolated by (NH4)2SO4 fractionation. Upon electrophoresis in the presence of sodium dodecylsulphate only four of the six bands that belong to the complex were found to have incorporated radioactivity; no radioactivity was found in the bands with an Mr of 40000 and 17000. The same result was obtained after labelling in the presence of acriflavin. If, however, 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 32000 band was now clearly depressed. 3. It is concluded that of the seven polypeptides of the cytochrome bc1 complex of yeast only one is made on mitochondrial ribosomes. This polypeptide has an Mr of 32000 and is probably associated with cytochrome b.