A VITAL FUNCTION FOR MITOCHONDRIAL-DNA IN THE PETITE NEGATIVE YEAST KLUYVEROMYCES-LACTIS

Petite-negative yeasts do not form viable respiratory-deficient mutant s on treatment with DNA-targeting drugs that readily eliminate the mit ochondrial DNA (mtDNA) from petite-positive yeasts. However, in the pe tite-negative yeast Kluyveromyces lactis, specific mutations in the nu clear genes MGI2 and MGI5 encoding the alpha- and gamma-subunits of th e mitochondrial F-1-ATPase, allow mtDNA to be lost. In this study we s how that wild-type K. lactis does not survive in the absence of its mi tochondrial genome and that the function of mgi mutations is to suppre ss lethality caused by loss of mtDNA. Firstly, we find that loss of a multicopy plasmid bearing a mgi allele readily occurs from a wild-type strain with functional mtDNA but is not tolerated in the absence of m tDNA. Secondly, we cloned the K. lactis homologue of the Saccharomyces cerevisiae mitochondrial genome maintenance gene MGM101, and disrupte d one of the two copies in a diploid. Following sporulation, we find t hat segregants containing the disrupted gene form mini-colonies contai ning 6-8000 inviable cells. By contrast, disruption of MGM101 is not l ethal in a haploid mgi strain with a specific mutation in a subunit of the mitochondrial F-1-ATPase. These observations suggest that mtDNA i n K. lactis encodes a vital function which may reside in one of the th ree mitochondrially encoded subunits of F-0.