The Role of RAD55 in Mitochondrial DNA Stability in Saccharomyces cerevisiae

Abstract

Mitochondria are organelles present in eukaryotic cells. Through the process of cellular respiration mitochondria produce ATP; a vital molecule for the completion of many cellular processes. Mitochondria are unique in that they contain their own DNA separate from the DNA within the nucleus. Mutations in mitochondrial DNA have notable connections to several human pathologies such as various neuromuscular and neurodegenerative disorders. The focus of this study was to determine the role of the nuclear gene RAD55 in maintaining mitochondrial DNA in budding yeast, Saccharomyces cerevisiae. The gene product of RAD55 cooperates with other proteins to bring about the repair of double-stranded DNA breaks in the nucleus. Specifically, RAD55 is a member of the RAD52 epistasis group whose gene product functions as a heterodimer with Rad57p. The Rad55p/57p heterodimer promotes Rad51p filament assembly on single-stranded DNA. Once assembled, Rad51p filaments displace Replication Protein A from single-stranded DNA and its recombinase activity is initiated. To determine the effect loss of Rad55p had on the stability of mitochondrial DNA, two genetic assays were performed. The first assay measured the frequency of spontaneous respiration loss in rad55? mutants. The lab observed rad55? mutants did not show a significant increase in spontaneous respiration loss compared to that of the wild type. An additional direct repeat-mediated deletion assay was performed to determine if Rad55p played a role in stabilizing the mitochondrial genome from mutations caused by recombination events. It was discovered that the rate of direct repeat-mediated deletions for rad55? in the nuclear genome increased 5.8-fold compared to that of the wild type. Surprisingly, the lab found the rate of direct repeat-mediated deletions for rad55? in the mitochondrial genome decreased by 1.5-fold compared to that of the wild type.