Identification of extragenic suppressors of the cif1 mutation in Saccharomyces cerevisiae.

The cif1 mutation of Saccharomyces cerevisiae causes inability to grow on glucose and related fermentable carbon sources. We have isolated two different suppressor mutations that allow growth on glucose of yeasts carrying the cif1 mutation. One of them, sci1-1, is recessive and caused inability to grow on non-fermentable carbon sources and to de-repress fructose-1,6-bisphosphatase. The other suppressor mutation, SCI2-1, is dominant and diminished the capacity to phosphorylate glucose or fructose. The SCI2-1 mutation decreased sporulation efficiency by 70% in heterozygosis and by more than 90% in homozygosis. In a CIF1 background, cells carrying the mutation SCI2-1 accumulated trehalose during the logarithmic phase of growth and hyperaccumulated it during the stationary phase. Genetic tests showed that SCI2 was either allelic, or else closely linked, to HXK2. The concentrations of the glycolytic metabolites measured during growth on glucose in cells carrying the cif1 mutation and any of the suppressor mutations were similar to those of a wild-type. Both types of suppressor mutations restored the transient cAMP response to glucose to cif1 mutants.