Cytochrome-c oxidase, the terminal complex of the mitochondrial respiratory chain that transfers electrons from cytochrome c to oxygen, has a critical role in cellular energy metabolism. In eukaryotes, the cytochrome-c oxidase complex is composed of from 7 to 13 subunits (in mammals), and its assembly depends on several nuclear-encoded proteins. The 0XA1 gene, which was first isolated in Saccharomyces cerevisiae, encodes a protein essential for cytochrome-c oxidase assembly. The human OXA1-like (OXA1L, previously designated OXA1Hs) cDNA was isolated by functional complementation of an oxa1{sup -} mutation in yeast. The deduced sequences of the two Oxa1 and Oxa1L proteins share 33% identity. Oxygen consumption measurements and cytochrome absorption spectra show that replacement of the yeast protein with the human homolog leads to the correct assembly of cytochrome-c oxidase, suggesting that these proteins play essentially the same role in both organisms. In this report, we have used both somatic cell hybrid mapping and in situ hybridization to localize the OXA1L gene on the human genome. 7 refs., 2 figs.