Saturation mutagenesis of the yeast his3 regulatory site: requirements for transcriptional induction and for binding by GCN4 activator protein

Expression of the yeast his3 and other amino acid biosynthetic genes is induced during conditions of amino acid starvation. The coordination of this response is mediated by a positive regulatory protein called GCN4, which binds specifically to regulatory sites upstream of all coregulated genes and stimulates their transcription. The nucleotide sequence requirements of the his3 regulatory site were determined by analysis of numerous point mutations obtained by a novel method of cloning oligonucleotides. Almost all single base pair mutations within the nine base pair sequence ATGACTCTT significantly reduce his3 induction in vivo and GCN4 binding in vitro, whereas changes outside this region have minimal effects. One mutation, which generates a sequence that most closely resembles the consensus for 15 coregulated genes, increases both the level of induction and the affinity for GCN4 protein. The palindromic nature of the optimal sequence, ATGACTCAT, suggests that GCN4 protein binds as a dimer to adjacent half-sites that possibly overlap.