CONSTITUTIVE AND CARBON SOURCE-RESPONSIVE PROMOTER ELEMENTS ARE INVOLVED IN THE REGULATED EXPRESSION OF THE SACCHAROMYCES-CEREVISIAE MALATESYNTHASE GENE MLS1

The malate synthase gene, MLS1, of the yeast Saccharomyces cerevisiae is transcriptionally regulated by the carbon source in the growth medi um. A MLS1-lacZ fusion gene, expressed at a basal level in the presenc e of 2% glucose, is derepressed more than 100-fold under conditions of sugar limitation. No evidence for MLS1 induction by oleic acid was fo und. By deletion analysis of the MLS1 control region, we identified tw o sites, UAS1 and UAS2, as important for efficient derepression of the gene. Both sites contain sequences that resemble the previously chara cterized carbon source-responsive element (CSRE) found in the promoter of the isocitrate lyase gene ICL1. Indeed, UAS1 and UAS2 in the MLSI upstream region turn out to be functional CSRE sequence variants. This finding allowed us to define a modified version of the CSRE consensus sequence (CCRTYSRNCCG). Protein binding to UAS1(MLS1) was observed wi th extracts from derepressed but not from repressed cells, and could b e competed for by an excess of the unlabelled CSRE(ICL1) sequence. No competition was observed with a mutated CSRE variant. Site-directed mu tagenesis of both CSREs in the MLSI promoter reduced gene activation u nder derepressing conditions to 20% of the wild-type level. The same d ecrease was observed with the wild-type MLSI promoter in a cat8 mutant , lacking an activator of CSRE-dependent transcription. The CSRE/Cat8p -independent activation of MLSI is mediated by constitutive UAS elemen ts. The pleiotropic transcription factor Abf1p, which binds to the MLS 1 upstream region, may contribute to constitutive activation. Thus, in order to ensure the severe glucose repression of MLSI observed, repre ssor elements that respond to the carbon source must counteract consti tutive activation. In summary, ICL1 and MLSI share common cis-acting e lements, although a distinct mechanism of carbon source control also c ontributes to MLSI regulation.