The nuclear gene PET112 was originally identified by a mutation (pet112-1) that specifically blocked accumulation of cytochrome c oxidase subunit II. The mutation causes a post-transcriptional defect since the level of COX2 mRNA in the mutant is the same as in the wild-type. However, PET112 does not have a function similar to that of PET111, a COX2 mRNA-specific translational activator: while pet111 mutations are suppressed by chimeric COX2 mRNAs bearing 5' leaders of other mitochondrial mRNAs, pet112-1 is not. The PET112 gene was isolated and shown to code a protein of 541 residues (62 kDa) with no significant homology to known amino-acid sequences. By hybridization to defined genomic clones the gene was mapped to chromosome II between cdc25 and ils1. Disruption of the PET112 open reading frame destabilized the mitochondrial genome, causing cells to become rho-. This finding suggests that PET112 has an important general function in mitochondrial gene expression, probably in translation.