Cytochrome b of cob revertants in yeast

22 revertants of Saccharomyces cerevisiae with intragenic suppressors ( sup 3 ) of cob exon mutations (G. Burger. Mol. Gen. Genet. , in the press) were analyzed. They display either a reduced amount of cytochrome b , or a shifted maximum absorption wavelength of total cytochrome b or a reduced growth rate on glycerol. The relationship of physico‐chemical properties (content, light absorption and midpoint potential of cytochromes b K and b T ) and functional properties (electron transport and energy yield) has been examined. In seven of eight revertants with a shifted maximum absorption wavelength of cytochrome b neither growth rate nor electron transfer activity was affected. In 13 of 14 revertants, reduced content of cytochrome b corresponds to a reduced electron transport rate through the cytochrome bc 1 segment. A lower enzymatic activity, which is not due to a quantitative but to a qualitative alteration of cytochrome b was found in two revertants. Two revertants show electron transport rates of wild‐type level concomitant with a reduced growth rate on glycerol, obviously due to a less efficient energy coupling. All revertants were shown to contain a high and a low potential cytochrome b , referred to as b K and b T . Those cob − /sup a mutations which shift the maximum absorption wavelength or diminish the content of cytochrome b affect both b cytochromes in all cases. The results support that electron transport and energy conservation are catalyzed by the unity of cytochrome b K and b T and that both heme centers are bound to an identical apoenzyme. Comparing electron flow rates of succinate:cytochrome c oxidoreductase and NADH:cytochrome c oxido‐reductase in cob mutants and two revertants provides evidence that ubiquinone does not constitute a homogeneous pool, suggested by the dissimilar interaction of both dehydrogenases with the bc 1 segment.