Identification of a third nuclear protein-coding gene required specifically for posttranscriptional expression of the mitochondrial COX3 gene is Saccharomyces cerevisiae.

A third nuclear protein-coding gene termed PET122 has been shown to be required for a post-transcriptional step in expression of the mitochondrial COX3 gene is Saccharomyces cerevisiae. pet122 mutants fail to produce cytochrome c oxidase subunit III, which is the polypeptide product of the COX3 gene, but produce normal amounts of mature COX3 mRNA. A strain bearing the pet122-1 allele is amber suppressible and correctly processes the 5' end of COX3 mRNA. Therefore, the PET122 gene product is a protein required for the expression of COX3 at some step after transcription and 5'-end processing of its transcript.