A 27 KDA PROTEIN BINDS TO A POSITIVE AND A NEGATIVE REGULATORY SEQUENCE IN THE PROMOTER OF THE ICL1 GENE FROM SACCHAROMYCES-CEREVISIAE

Isocitrate lyase, encoded by ICL1, is one of the key enzymes of the gl yoxylate pathway, which operates as an anaplerotic route for replenish ing the tricarboxylic acid cycle; it is required for growth of Sacchar omyces cerevisiae on carbon sources such as ethanol, but is dispensabl e when fermentable carbon sources are available, The positive regulati on of the ICL1 gene by an upstream activating sequence (UAS) element l ocated between -397 and -388 has been previously reported. In this pap er we show that the ICL1 promoter sequence 5'-AGTCCGGACTAGCATCCCAG-3' located between -261 and -242 contains an upstream repressing sequence (URS) element. We have identified and partially purified a 27 kDa pro tein that binds specifically to both the UAS and URS sequences of the ICL1 promoter. For both UAS and URS, binding requires the protein Snf1 (Cat1), a protein kinase essential for the derepression of genes repr essed by glucose. Binding does not take place with extracts from gluco se-grown strains, unless they lack Mig1, a negative regulatory protein involved in glucose repression.