IRC3 Regulates Mitochondrial Translation in Response to Metabolic Cues in Saccharomyces cerevisiae

Mitochondrial oxidative phosphorylation (OXPHOS) enzymes have a dual genetic origin. Mechanisms regulating the expression of nucleus-encoded OXPHOS subunits in response to metabolic cues (glucose versus glycerol) are well understood, while the regulation of mitochondrially encoded OXPHOS subunits is poorly defined. Here, we show that IRC3, a DEAD/H box helicase gene, previously implicated in mitochondrial DNA maintenance, is central to integrating metabolic cues with mitochondria! translation. Irc3 associates with mitochondrial small ribosomal subunits in cells consistent with its role in regulating translation elongation based on the Arg8(m) reporter system. IRC3-deleted cells retained mitochondrial DNA despite a growth defect on glycerol plates. Glucose-grown Delta irc3 rho(+) and irc3 temperature-sensitive cells at 37 degrees C have reduced translation rates from the majority of mRNAs. In contrast, when galactose was the carbon source, a reduction in mitochondrial translation was observed predominantly from Cox1 mRNA in Delta irc3 rho(+) cells but no defect was observed in irc3 temperature-sensitive cells, at 37 degrees C. In support of a model whereby IRC3 responds to metabolic cues to regulate mitochondrial translation, Delta irc3 rho(+) suppressor strains isolated for restoration of growth on glycerol medium restore mitochondrial protein synthesis differentially in the presence of glucose versus glycerol.