Locus-specific suppression of ilv1 in Saccharomyces cerevisiae by deregulation of CHA1 transcription.

The ILV1 gene of Saccharomyces cerevisiae encodes the anabolic threonine deaminase, which catalyzes the first committed step in isoleucine biosynthesis. Strains devoid of a functional Ilv1p have a requirement for isoleucine. Threonine can also be deaminated by a second serine/threonine deaminase encoded by the CHA1 gene. CHA1 is regulated by transcriptional induction by serine and threonine, and enables yeast to utilize the hydroxyamino acids as sole nitrogen source. Phenotypic suppression of ilv1 can occur by inducer-mediated transcriptional activation of the CHA1 gene. To identify mutations in putative trnas-acting factors regulating CHA1 expression, we have isolated and characterized three extragenic suppressors of ilv1. A dominant mutation, SIL4 (suppressor of ilv1), is allelic to HOM3. It increases the size of the threonine pool, by 15- to 20-fold, which is sufficient to induce CHA1 transcription, thereby creating a metabolic bypass of ilv1. A second dominant mutation, SIL3, and a recessive mutation, sil2, both suppress ilv1 by causing inducer-independent, constitutive transcription of CHA1. Importantly, sil2 and SIL3 increase the expression of a CHA1p-lacZ translational gene fusion, demonstrating that they exert their action through the CHA1 promoter. Genetic analysis showed that both SIL3 and sil2 are alleles of CHA4, a positive regulator of CHA1, i.e., they convert Cha4p to a constitutive activator.