Nucleotide sequence of cytochrome oxidase (subunit III) from the mitochondrion of the tunicate Pyura stolonifera: evidence that AGR encodes glycine

The mitochondrial gene encoding cytochrome oxidase subunit III (CO III) of the tunicate Pyura stolonifera was cloned on 2 overlapping fragments (a 3 kb BamHI fragment and a 2.3 kb EcoRI/XhoI fragment) of mitochondrial DNA and identified by homology to previously reported CO III sequences of other species. The sequence is 52% and 51% identical to human CO III at the nucleotide and amino-acid levels, respectively. Codon usage in mitochondria deviates from the "universal" genetic code of nuclear genes. In particular, AGA and AGG (AGR), which code for arginine in nuclear genes, are known to code for serine in non-vertebrate metazoans (2 degree 4), and, when present, are termination codons in vertebrates.