Regulatory Interactions between Mitochondrial Genes: Expressed Introns—Their Function and Regulation

Abstract

The mitochondrial gene for cytochrome b (cob) (Tzagoloff et al. 1976) in Saccharomyces cerevisiae and related strains exhibits a mosaic organization (Fig. 1). In some, so-called short-form strains, it contains three exons and two introns, whereas in others, so-called long-form strains, it contains six exons (B1–B6) and five introns (I1–I5) (Grivell et al. 1980; Jacq et al. 1980; Nobrega and Tzagoloff 1980). In this paper we consistently number introns and exons relative to the long form of the gene (cob^L), since it is clear that the short form (cob^S) is missing I1–I3 and that the first four exons of cob^L are fused to form the first reading frame of cob^S. Three of the introns (I2, I3, and I4) specify discrete trans-acting elements (Lamouroux et al. 1980) required for the correct processing (splicing) of the primary transcript (consisting of all sequences of the gene proper plus a leader of about 1000 bases) into the mature mRNA, in which all exon sequences have been fused and all introns have been eliminated (Halbreich et al. 1980; Haid et al. 1981; Van Ommen et al. 1980). Mutants in these introns exhibit a pleiotropic phenotype. They not only lack apocytochrome b but are also deficient in the expression of cytochrome oxidase subunit I (Jacq et al. 1980; Dhawale et al. 1981), itself the product of the oxi3 gene (Bonitz et al. 1980). Consistent with these deficiencies, they accumulate processing intermediates in transcripts of both genes. In addition, they also accumulate novel polypeptides...