Mitochondria contain their own genetic system to express a small number of hydrophobic polypeptides, including cytochrome b, an essential subunit of the bc(1) complex of the respiratory chain. In this paper, we show in yeast that Cbp3, a bc(1) complex assembly factor, and Cbp6, a regulator of cytochrome b translation, form a complex that associates with the polypeptide tunnel exit of mitochondrial ribosomes and that exhibits two important functions in the biogenesis of cytochrome b. On the one hand, the interaction of Cbp3 and Cbp6 with mitochondrial ribosomes is necessary for efficient translation of cytochrome b transcript [corrected]. On the other hand, the Cbp3-Cbp6 complex interacts directly with newly synthesized cytochrome b in an assembly intermediate that is not ribosome bound and that contains the assembly factor Cbp4. Our results suggest that synthesis of cytochrome b occurs preferentially on those ribosomes that have the Cbp3-Cbp6 complex bound to their tunnel exit, an arrangement that may ensure tight coordination of cytochrome b synthesis and assembly.
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