Mosaic organization of a mitochondrial gene: evidence from double mutants in the cytochrome b region of Saccharomyces cerevisiae

The region coding for apocytochrome b in the mitochondrial genome of Saccharomyces cerevisiae is believed to exhibit a mosaic organization, consisting in certain strains of five exons and four introns. This model can be tested by the use of double mutants, each containing two physically, genetically and phenotypically defined mit- lesions in cis, (that is, in the same mitochondrial chromosome). Such mutants have been constructed, and the phenotypes of several examples of each of the four possible classes--exon-exon, exon-intron (downstream), intron (upstream)-exon and intron-intron--have been examined. Our results have shown that upstream mutations are always epistatic to downstream ones for polypeptide products, and that regulation of expression of cytochrome oxidase subunit I by introns is epistatic regardless of position. These findings have provided an independent verification of the mosaic model, and also suggest that at least the majority of novel polypeptides accumulating in intron mutants are hybrid products that contain sequences of the wild-type polypeptide.