Derepression of citrate synthase in Saccharomyces cerevisiae may occur at the level of transcription.

Pulse-chase labeling in whole cells and cell-free protein synthesis were used to establish that the mitochondrial enzyme citrate synthase is made as a larger precursor in Saccharomyces cerevisiae. A 54,000 Mr precursor form appeared to be a primary translation product since it could be labeled with N-[35S]formylmethionine in vitro. The induction of citrate synthase was monitored in S. cerevisiae cells grown on fermentable (glucose) and nonfermentable (ethanol and glycerol) carbon sources. The amount of citrate synthase activity and immune-reactive protein increased more than 15-fold as S. cerevisiae cells entered the stationary growth phase on glucose-containing medium. This increase was paralleled by an increase in translatable RNA for the enzyme. When cells were grown on a nonfermentable carbon source, no increase in either citrate synthase or its mRNA was detected. The results suggest that the release of citrate synthase from catabolite repression may occur at the level of transcription.