Structure of the Apocytochrome-b Gene and Processing of Apocytochrome-b Transcripts in Saccharomyces cerevisiae

Abstract

Cytochrome b is a catalytic subunit of the coenzyme QH₂–cytochrome c reductase complex of the mitochondrial respiratory chain. Although there are two distinct cytochromes b (b₅₆₂ and b₅₆₆) associated with this complex, biochemical and genetic evidence indicates the existence of only one polypeptide with two separate heme-binding sites (Tzagoloff and Nobrega 1980). The spectral properties and redox potentials that distinguish cytochromes b₅₆₂ and b₅₆₆ (Chance et al. 1970) can therefore be most easily rationalized in terms of two different environments of the heme groups in the protein.

Cytochrome b is the only subunit of the coenzyme QH₂–cytochrome c reductase complex synthesized on mitochondrial ribosomes (Weiss et al. 1973; Katan et al. 1976). Earlier studies of mit⁻ mutants of yeast confirmed that the protein is also encoded in mtDNA (Tzagoloff et al. 1975; Slonimski et al. 1978; Haid et al. 1979). The complete sequences of the yeast gene (Nobrega and Tzagoloff 1980), bovine gene, and human gene (Anderson et al. 1981) have revealed that, although the proteins are highly homologous, the gene itself has undergone extensive evolutionary change. This is most dramatically illustrated by the presence of intervening sequences in the fungal genes and by their absence in the mammalian genes.

In this paper we describe the gene structure and some aspects of the processing of the apocytochrome-b messenger in Saccharomyces cerevisiae strain D273-10B. Of the yeast strains studied (Dhawale et al. 1981), S. cerevisiae D273-10B has the least complex gene organization, thereby making the analysis of the apocytochrome-b...