ADDITIVE ACTIVATION OF YEAST LEU4 TRANSCRIPTION BY MULTIPLE CIS-ELEMENTS

The LEU4 gene of Saccharomyces cerevisiae and the enzyme encoded by LEU4, alpha-isopropylmalate synthase, occupy a special position in amino acid metabolism. alpha-Isopropylmalate synthase catalyzes the first committed step in leucine biosynthesis. However, the reaction product alpha-isopropylmalate is not only an intermediate in the leucine biosynthetic pathway, but also functions as co activator of at least six genes, both within and outside of the leucine pathway. The metabolic importance of alpha-isopropylmalate appears to be reflected in the surprisingly multifaceted regulation of LEU4 expression. This report describes an analysis of functional cis elements in the LEU4 promoter. Five such elements were identified. Three distal elements, designated UAS(LEU), GCE-A, and GCE-B, are responsible for regulation by the regulatory proteins Leu3p and Gcn4p, respectively. The incremental activation of LEU4 by these elements is additive and independent. In addition, two proximal elements were localized. One of these con forms to the TATA consensus sequence and exhibits high affinity for TATA binding protein. The other element shows strong sequence identity with the Bas2p binding site and appears to be involved in basal and phosphate-mediated regulation of LEU4.