A vital function for mitochondrial DNA in the petite negative yeast Kluyveromyces lactis

Petite-negative yeasts do not form viable respiratory-deficient mutants on treatment with DNA-targeting drugs that readily eliminate the mitochondrial DNA (mtDNA) from petite-positive yeasts. However, in the petite-negative yeast Kluyveromyces lactis, specific mutations in the nuclear genes MGI2 and MGI5 encoding the alpha- and gamma-subunits of the mitochondrial F-1-ATPase, allow mtDNA to be lost. In this study we show that wild-type K. lactis does not survive in the absence of its mitochondrial genome and that the function of mgi mutations is to suppress 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 mtDNA. Secondly, we cloned the K. lactis homologue of the Saccharomyces cerevisiae mitochondrial genome maintenance gene MGM101, and disrupted one of the two copies in a diploid. Following sporulation, we find that segregants containing the disrupted gene form mini-colonies containing 6-8000 inviable cells. By contrast, disruption of MGM101 is not lethal in a haploid mgi strain with a specific mutation in a subunit of the mitochondrial F-1-ATPase. These observations suggest that mtDNA in K. lactis encodes a vital function which may reside in one of the three mitochondrially encoded subunits of F-0.