Expression of petite mitochondrial DNA in vivo: zygotic gene rescue.

A protocol is introduced for probing the organization and regulation of expression of the yeast mitochondrial genome, termed "zygotic gene rescue." The procedure is based on the notion that genes retained on mitochondrial DNA of on the notion that genes retained on mitochondrial DNA of petites can be expressed in zygotes of a cross between petite and wild type. To test the validity of this notion, we have taken advantage of our ability to discriminate, by mobility differences on sodium dodecyl sulfate/polyacrylamide gels, different forms of the product of alleles of the mitochondrial gene, varI. In petite strains that have retained the varI gene, its characteristic product appears in zygotes 4-5 hr after mating; no product is observed in petite strains deleted in the varI locus. Our studies indicate that (i) expression in the zygote of the varI gene in the petite genome is not exclusively the result of recombination with mitochondrial DNA of the wild-type tester, and (ii) the varI gene is probably reiterated in the petite mitochondrial genome. The strength of the technique of zygotic gene rescue in the analysis of the mitochondrial genome is discussed.