Nrg1 is a transcriptional repressor for glucose repression of STA1 gene expression in Saccharomyces cerevisiae

Expression of genes encoding starch-degrading enzymes is regulated by gluco se repression in the yeast Saccharomyces cerevisiae, We have identified a t ranscriptional repressor, Nrg1, in a genetic screen designed to reveal nega tive factors involved in the expression of STA1, which encodes a glucoamyla se. The NRG1 gene encodes a 25-kDa C2H2 zinc finger protein which specifica lly binds to two regions in the upstream activation sequence of the STA1 ge ne, as judged by gel retardation and DNase I footprinting analyses. Disrupt ion of the NRG1 gene causes a fivefold increase in the level of the STA1 tr anscript in the presence of glucose. The expression of NRG1 itself is inhib ited 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 repress ion is abolished in both ssn6 and tap1 mutants. Two-hybrid and glutathione S-transferase pull-down experiments show an interaction of Nrg1 with Ssn6 b oth in vivo and in vitro. These findings indicate that Nrg1 acts as a DNA-b inding repressor and mediates glucose repression of the STA1 gene expressio n by recruiting the Ssn6-Tup1 complex.