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

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.In yeast, a large number of genes are turned off during growth on glucose (9, 37, 49). These glucose-repressible genes can be divided into three groups: (i) genes for metabolizing other carbon sources; (ii) genes encoding enzymes unique to gluconeogenesis; and (iii) genes involved in the Krebs cycle and in respiration. The Mig1 glucose repressor is a zinc finger protein and binds to the GC-rich motif identified in the promoters of several glucose-repressed genes, including the GAL1, GAL4, SUC2, and MAL genes (10,13,28,29). In the absence of glucose, the Snf1 kinase inhibits the function of Mig1 protein directly or indirectly, leading to derepression of glucose-repressed genes (3, 4). Nuclear translocation of Mig1 is regulated by differential phosphorylation of the protein in response to glucose availability, and recruitment of the general repression complex Ssn6-Tup1 to the DNA-bound Mig1 is required for the repression (5,17,48). Disruption of the MIG1 gene, however, only partially relieves glucose repression of SUC2 and has little or no effect on glucose repression of other genes whose promoters contain the Mig1-binding sites (27,31,37,50), indicating the involvement of other repressors in glucose repression. For instance, Mig2 was recently identified as a second repressor responsible for the remaining glucose repression of SUC2 and contains zinc fingers very similar to those of Mig1 (24).In Saccharomyces cerevisiae var. diastaticus, three unlinked homologous STA genes (STA1, STA2, and STA3) encode glucoamylase isozymes (GAI, GAII, and GAIII), which are responsible for enzymatic degradation of starch to glucose (16,22,25,32,35,47,52). Expression of the STA genes is regulated by complex interactions between positive and negative factors and their cognate elements (1,19,21,33,41). The negative regulation occurs at three different levels: (i) carbon catabolite repression...

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