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The level of core protein I and subunit VI of mitochondrial complex III (which are coded by the nuclear genome) was found to be greatly diminished in a yeast strain carrying a mutation (W7) in the mitochondrial gene coding for cytochrome b. This suggests that intricate interactions occur in complex III biogenesis between proteins of cytoplasmic and mitochondrial origin. This mutant was characterized by a low cytochrome b level and a loss of activity in the b-c1 segment of the respiratory chain. It was compared to another mutant showing similar biochemical characteristics, but which had integrated core protein I, as shown by antibody binding experiments. In mutant devoid of core protein I, cytochrome b was found to be reducible by NADH but not by succinate, suggesting two different electron transfer pathways inside comples III from each substrate to cytochrome b heme(s).

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