Allele-specific expression of the Mgi^- phenotype on disruption of the F₁-ATPase delta-subunit gene in Kluyveromyces lactis

Kluyveromyces lactis is a petite-negative yeast that does not form viable mitochondrial genome-deletion mutants (petites) when treated with DNA-targeting drugs. Loss of mtDNA is lethal for this yeast but mutations at three loci termed MGI, for mitochondrial genome integrity, can suppress this lethality. The three loci encode the α-, β- and γ-subunits of mitochondrial F₁-ATPase. In this study we report the isolation and characterization of the KlATPδ gene encoding the δ-subunit of F₁-ATPase. The deduced protein contains 158 amino acids showing 72% identity to the protein from Saccharomyces cerevisiae and a putative mitochondrial targeting sequence of 23 amino acids. Disruption of the gene causes cells to become respiratory deficient while the introduction of ATPδ from S. cerevisiae restores growth on glycerol. Cells with a disrupted ATPδ gene, like strains with disruptions of α-, β- and γ-F₁-subunits, do not produce petite mutants when treated with ethidium bromide. However, unlike strains with disruptions in the three largest F₁-subunits, disruption of ATPδ in the presence of some mgi alleles does not abolish the Mgi^- phenotype. By contrast, elimination of ATPδ in other mpi strains removes resistance to ethidium bromide and ρ⁰ mutants are not formed. Hence the ATPδ subunit of F₁-ATPase, while not mandatory for a Mgi^- phenotype, aids some mgi alleles in suppressing ρ⁰ lethality.