Characterization of COX9, the nuclear gene encoding the yeast mitochondrial protein cytochrome c oxidase subunit VIIa. Subunit VIIa lacks a leader peptide and is an essential component of the holoenzyme.

The gene COX9 for subunit VIIa of cytochrome c oxidase from Saccharomyces cerevisiae has been cloned with the aid of an oligonucleotide probe. From the nucleotide sequence of COX9, we deduce that subunit VIIa is derived from a precursor that is 59 amino acids in length (Mr = 6963). This precursor is longer than mature subunit VIIa by one amino acid at its NH2 terminus and four amino acids at its COOH terminus. COX9 exists as a single copy in the haploid genome of S. cerevisiae and produces one major transcript. When the genomic copy of COX9 is removed, cells lack a functional cytochrome c oxidase holoenzyme. From the predicted secondary structure of subunit VIIa, previous data concerning its relationship to the lipid bilayer of the inner membrane and the location of its hydrophobic domains (Power, S.D., Lochrie, M.A., and Poyton, R.O. (1986) J. Biol. Chem. 261, 9206-9209) and the finding that it is essential for the holoenzyme, we propose a model for subunit VIIa which suggests that this small integral protein plays a role in holoenzyme assembly or stability.