Translational regulation of mitochondrial gene expression by nuclear genes of Saccharomyces cerevisiae.

We describe several yeast nuclear mutations that specifically block expression of the mitochondrial genes encoding cytochrome c oxidase subunits II (COXII) and III (COXIII). These recessive mutations define positive regulators of mitochondrial gene expression that act at the level of translation. Mutations in the nuclear gene PET111 completely block accumulation of COXII, but the COXII mRNA is present in mutant cells at a level approximately one-third of that of the wild type. Mitochondrial suppressors of pet111 mutations correspond to deletions in mtDNA that result in fusions between the coxII structural gene and other mitochondrial genes. The chimeric mRNAs encoded by these fusions are translated in pet111 mutants; this translation leads to accumulation of functional COXII. The PET111 protein probably acts directly on coxII translation, because it is located in mitochondria. Translation of the mitochondrially coded mRNA for COXIII requires the action of at least three nuclear genes, PET494, PET54 and a newly discovered gene, provisionally termed PET55. Both the PET494 and PET54 proteins are located in mitochondria and therefore probably act directly on the mitochondrial translation system. Mutations in all three genes are suppressed in strains that contain chimeric coxIII mRNAs with the 5'-untranslated leaders of other mitochondrial transcripts fused to the coxIII coding sequence. The products of all three nuclear genes may form a complex and carry out a single function.(ABSTRACT TRUNCATED AT 250 WORDS)