Maintenance of the mitochondrial genome in Saccharomyces cerevisiae: Role of replication and recombination proteins

The mitochondrial genome encodes protein components of the oxidative phosphorylation machinery in a cell. Disruption of mitochondrial genome integrity results in impaired respiration capacity that can eventually lead to cell death. Point mutations and deletions in the mitochondrial DNA are associated with numerous mitochondrial myopathies, cancers and age related tissue degeneration in humans. Thus, maintenance of mitochondrial genome by employing efficient DNA replication and repair systems is essential for cell survival. In this study I designed and characterized a genetic reporter to measure repeat-mediated deletions in the mitochondrial DNA of the budding yeast, Saccharomyces cerevisiae and used genetic screens to identify proteins responsible for maintenance of the mitochondrial DNA. Using the mitochondrial specific genetic reporter, I identified a role for nuclear homologous recombination proteins of the RAD52 epistasis group in generating deletions in the mitochondrial genome. Using a respiration specific synthetic lethal screen, I identified a mitochondrial DNA recombination suppressor, Irc3p. Mutations in IRC3 result in an increase in mitochondrial DNA recombination and sensitivity to ultra-violet light. IRC3 mutant cells allow visualization of fluorescently labeled Rad52p in the mitochondrial compartment and the detection of Rad52p binding to mitochondrial DNA. We hypothesize that Irc3p functions in mitochondrial DNA repair. Abolishing the function of the Irc3 protein results in an increase in mitochondrial DNA double strand breaks. These breaks can be bound by the Rad52 protein in an attempt to repair the DNA strand breaks via recombination dependent mechanisms. Using in vitro and in vivo assays I also find that the putative cell surface protein Sed1p physically interacts with the mitochondrial DNA polymerase, Mip1p. Deletion of the SED1 gene increases mitochondrial DNA instability likely by affecting the protein levels of Mip1p within the mitochondrial compartment. Together, these studies have revealed the roles of previously uncharacterized nuclear encoded mitochondrial proteins in preservation and protection of the mitochondrial genome.