Expression of the "split gene" COB in yeast mtDNA. Translation of intervening sequences in mutant strains.

This study deals with the effects that mutations in the COB region of yeast mtDNA have on the expression of mitochondrially made polypeptides. Based on the detection of two series of polypeptide chain-terminating mutations, we conclude that two proteins are specified by this region, apocytochrome b (Mr = 30,000) and a polypeptide of Mr = 42,000. One series of mutations generates new polypeptides ranging in size from 8,000 to 29,000 daltons; all of them are precipitated by serum direct against apocytochrome b. These mutations are located in five distinct segments of the COB region, the sequences alpha to epsilon coding for apocytochrome b. The second series of mutations, generating new polypeptides ranging in size from 17,000 to 41,000 daltons, is located within the first intervening sequence (alpha/beta) of the split gene for apocytochrome b. These mutations cause premature chain termination in the COOH-terminal part of a 42,000-dalton polypeptide. Its NH2-terminal part is likely to be specified by sequence alpha and thus to be homologous to that of apocytochrome b. We conclude that the 42,000-dalton polypeptide is translated on a processing intermediate of th COB transcript by reading through sequence alpha into sequence alpha/beta. We discuss the hypothesis that this polypeptide has a function in the expression of the COB region, possibly at the level of transcript processing.