Expression of the SUC2 gene of Saccharomyces cerevisiae is induced by low levels of glucose

High levels of glucose repress expression of the SUC2 gene in the yeast Saccharomyces cerevisiae. We have discovered that low levels of glucose are required for maximal transcription of SUC2: SUC2 expression is induced about five- to ten-fold in cells growing on low levels of glucose (0.1%) compared to cells growing on galactose or glycerol. Two pieces of evidence suggest that this low-glucose-induced expression is mediated by a repression mechanism that involves an upstream repression site in the SUC2 promoter (URS(SUC2)). First, deletion of the URS(SUC2) results in expression of the SUC2 gene in the absence of glucose, and second the URS(SUC2) mediates a six-fold repression of a reporter gene when inserted into a heterologous promoter. However, this URS(SUC2)-mediated repression occurs on all tested carbon sources, suggesting that this URS element acts in concert with all other promoter elements to respond to low concentrations of glucose. This repression requires the general repressor Ssn6p. SNF3, which encodes a glucose transporter that appears to be a sensor of low levels of glucose, is also required for low-glucose-induced expression of SUC2.