Mitochondrial ATP synthase subunit 9 is not required for viability of the petite-negative yeast Kluyveromyces lactis.

Specific mutations in nuclear MGI genes encoding the alpha, beta and gamma subunits of the mitochondrial inner membrane F1-ATPase complex allow mitochondrial DNA (mtDNA) to be lost from K. lactis. In the absence of a mutation in any of these three nuclear genes, loss of mtDNA is lethal. These results imply that mtDNA encodes a gene that is essential. Likely candidates for such an essential role are the ATP6, 8 and 9 genes coding for proteins of the ATP synthase-F0 component. The present study removes ATP9 from contention as a vital mitochondrial gene because in a respiratory deficient mutant, Gly- 3. 9, lacking a nuclear mgi mutation, we have found that a rearrangement in mtDNA has deleted 22 amino acids from the carboxy terminus of the 75 amino-acid subunit-9 protein. Rearrangement in mtDNA has occurred by recombination at a 23-bp repeated sequence in the introns of the ATP9 and large ribosomal RNA (LSU) subunit genes. These two introns, of 394 (ATP9) and 410 (LSU) nucleotides, both belong to group 1.