Nrg1 Is a Transcriptional Repressor for Glucose Repression of STA1 Gene Expression in Saccharomyces cerevisiae

Abstract

ABSTRACT Expression of genes encoding starch-degrading enzymes is regulated by glucose repression in the yeast Saccharomyces cerevisiae . We have identified a transcriptional repressor, Nrg1, in a genetic screen designed to reveal negative factors involved in the expression of STA1 , which encodes a glucoamylase. The NRG1 gene encodes a 25-kDa C 2 H 2 zinc finger protein which specifically binds to two regions in the upstream activation sequence of the STA1 gene, as judged by gel retardation and DNase I footprinting analyses. Disruption of the NRG1 gene causes a fivefold increase in the level of the STA1 transcript in the presence of glucose. The expression of NRG1 itself is inhibited in the absence of glucose. DNA-bound LexA-Nrg1 represses transcription of a target gene 10.7-fold in a glucose-dependent manner, and this repression is abolished in both ssn6 and tup1 mutants. Two-hybrid and glutathione S -transferase pull-down experiments show an interaction of Nrg1 with Ssn6 both in vivo and in vitro. These findings indicate that Nrg1 acts as a DNA-binding repressor and mediates glucose repression of the STA1 gene expression by recruiting the Ssn6-Tup1 complex.