Significantly reduced, but balanced, rates of mitochondrial fission and fusion are sufficient to maintain the integrity of yeast mitochondrial DNA.

Mitochondria exist as dynamic tubular networks and the morphology of these networks impacts organelle function and cell health. Mitochondrial morphology is maintained in part by the opposing activities of mitochondrial fission and fusion. Mitochondrial fission and fusion are also required to maintain mitochondrial DNA (mtDNA) integrity. In Saccharomyces cerevisiae, the simultaneous inhibition of mitochondrial fission and fusion results in increased mtDNA mutation and the consequent loss of respiratory competence. The mechanism by which fission and fusion maintain mtDNA integrity is not fully understood. Previous work demonstrates that mtDNA is spatially linked to mitochondrial fission sites. Here, we extend this finding using live-cell imaging to localize mtDNA to mitochondrial fusion sites. While mtDNA is present at sites of mitochondrial fission and fusion, mitochondrial fission and fusion rates are not altered in cells lacking mtDNA. Using alleles that alter mitochondrial fission and fusion rates, we find that mtDNA integrity can be maintained in cells with significantly reduced, but balanced, rates of fission and fusion. In addition, we find that increasing mtDNA copy number reduces the loss of respiratory competence in double mitochondrial fission-fusion mutants. Our findings add novel insights into the relationship between mitochondrial dynamics and mtDNA integrity.