LOCUS-SPECIFIC SUPPRESSION OF ILV1 IN SACCHAROMYCES-CEREVISIAE BY DEREGULATION OF CHA1 TRANSCRIPTION

The ILV1 gene of Saccharomyces cerevisiae encodes the anabolic threoni ne deaminase, which catalyzes the first committed step in isoleucine b iosynthesis. Strains devoid of a functional Ilvlp have a requirement f or isoleucine. Threonine can also be deaminated by a second serine/thr eonine deaminase encoded by the CHA1 gene. CHA1 is regulated by transc riptional induction by serine and threonine, and enables yeast to util ize the hydroxyamino acids as sole nitrogen source. Phenotypic suppres sion of ilv1 can occur by inducer-mediated transcriptional activation of the CHA1 gene. To identify mutations in putative trans-acting facto rs regulating CHA1 expression, we have isolated and characterized thre e extragenic suppressors of ilv1. A dominant mutation, SIL4 (suppresso r 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 transcrip tion, thereby creating a metabolic bypass of ilv1. A second dominant m utation, SIL3, and a recessive mutation, sil2, both suppress ilv1 by c ausing inducer-independent, constitutive transcription of CHA1. Import antly, sil2 and SIL3 increase the expression of a CHAlp-lacZ translati onal gene fusion, demonstrating that they exert their action through t he CHA1 promoter. Genetic analysis showed that both SIL3 and sil2 are alleles of CHA4, a positive regulator of CHA1, i.e., they convert Cha4 p to a constitutive activator.