Mutations in the Saccharomyces cerevisiae ATP22 gene were previously shown to block assembly of the F 0 component of the mitochondrial proton-translocating ATPase. Further inquiries into the function of Atp22p have revealed that it is essential for translation of subunit 6 of the mitochondrial ATPase. The mutant phenotype can be partially rescued by the presence in the same cell of wild-type mitochondrial DNA and a r À deletion genome in which the 59-UTR, first exon, and first intron of COX1 are fused to the fourth codon of ATP6. The COX1/ATP6 gene is transcribed and processed to the mature mRNA by splicing of the COX1 intron from the precursor. The hybrid protein translated from the novel mRNA is proteolytically cleaved at the normal site between residues 10 and 11 of the subunit 6 precursor, causing the release of the polypeptide encoded by the COX1 exon. The ability of the r À suppressor genome to express subunit 6 in an atp22 null mutant constitutes strong evidence that translation of subunit 6 depends on the interaction of Atp22p with the 59-UTR of the ATP6 mRNA.