Regulatory sites for his3 gene expression in yeast

Abstract

The expression levels of many genes are regulated in response to particular environmental or developmental cues. A simple regulated gene can conceptually be divided into three elements: structural components that encode the gene product, promoter elements that are essential for gene expression, and regulatory elements responsible for changing the level of expression after a specific stimulus^1,2. When cells of the yeast Saccharomyces cerevisiae are subjected to amino acid starvation, his3 and many other amino acid biosynthetic genes are expressed threefold above the basal level^3,4. Previously, I isolated mutations mapping outside the structural gene that severely reduce or eliminate his3 expression^5,6. These mutations define two distinct his3 promoter elements located 115–155 and 32–52 base pairs (bp) from the site of transcriptional initiation. Here I describe 18 small deletion mutations, some of which express his3 at the basal level but are unable to increase the level of expression in the appropriate physiological conditions. These define two regulatory sites located 32–41 and 80–100 bp upstream from the site of transcriptional initiation, and they strongly suggest that these regions are necessary for the positive regulation of his3 expression. I consider the results in terms of models for the general control of amino acid biosynthesis.