Neo-functionalization inSaccharomyces cerevisiae: A Novel Nrg1-Rtg3 chimeric transcriptional modulator is essential to maintain mitochondrial DNA integrity

Abstract

AbstractInSaccharomyces cerevisiae, the transcriptional repressor Nrg1 (Negative Regulator of Glucose-repressed genes) and the b/Zip transcription factor Rtg3 (ReTroGrade regulation) mediate glucose repression and mitochondria to nucleus signaling, respectively. Here we show a novel function for these two proteins, in which alanine promotes the formation of a chimeric Nrg1/Rtg3 regulator that represses theALT2gene (encoding an alanine transaminase paralogue of unknown function) expression. ANRG1/NRG2paralogous pair, resulting from a post-wide genome, small scale duplication event, is extant in theSaccharomycesgenus. Neo-functionalization of only one paralogue resulted in Nrg1, able to interact with Rtg3. Eithernrg1Δ orrtg3Δ single mutant strains are unable to utilize ethanol and show a typical petite (small) phenotype on glucose. Neither of the WT genes complemented the petite phenotype, suggesting irreversible mitochondrial DNA damage in these mutants. Neithernrg1Δ norrtg3Δ mutant strains express genes encoded by any of five polycistronic units transcribed from mitochondrial DNA inS. cerevisiae. This, and the direct measure of the mitochondrial DNA gene complement confirms that irreversible damage of the mitochondrial DNA occurred in both mutant strains and is consistent with an essential role of the chimeric Nrg1/Rtg3 regulator in mitochondrial DNA maintenance.