The yeast nuclear gene CBS1 is required for translation of mitochondrial mRNAs bearing the cob 5' untranslated leader.

Mitochondrial translation of the cob mRNA to yield apocytochrome b is specifically dependent on the nuclear gene CBS1, while mitochondrial translation of the oxi2 mRNA to yield cytochrome oxidase subunit III (cox III) is specifically dependent on the nuclear gene PET494. Chimeric oxi2 mRNAs bearing the 5' leaders of other mitochondrial mRNAs, transcribed from rho- mitochondrial DNAs termed MSU494, are translated in pet494 mutants. In this study, we examined translation of coxIII from MSU494-encoded chimeric mRNAs in zygotes of defined nuclear and mitochondrial genotype. CoxIII was translated from a chimeric mRNA bearing the cob leader only when the zygotes contained a wild-type CBS1 gene. CoxIII translation from an mRNA bearing the 5' leader of the mitochondrial gene aap1 was not dependent on CBS1 activity. We conclude that the product of the nuclear gene CBS1, or something under its control, acts in the mitochondrion on the cob mRNA 5' leader to activate translation of down-stream coding sequences.