Abstract

Glucose is the major carbon and energy source for most cells and the first rate‐limiting step of its utilization, its uptake, is facilitated by glucose transporters. Expression of several glucose transporter (HXT) genes in the budding yeast Saccharomyces cerevisiae is repressed by the HXT repressor Rgt1, which recruits glucose‐responsive transcription factor Mth1 and the general corepressor complex Ssn6‐Tup1 in the absence of glucose; however, it is derepressed when Mth1 is inactivated by glucose. We sow evidence that Ssn6‐Tup1 interferes with the DNA‐binding ability of Rgt1 in the absence of Mth1 and Ssn6 overexpression abolishes the Rgt1 function which is restored by co‐overexpression of Mth1. Thus Mth1 functionally antagonizes Ssn6‐Tup1 and regulates Rgt1 function not by directly modulating its DNA‐binding activity.Mth1 acts as a scaffold‐like protein to recruit Ssn6‐Tup1 to Rgt1. Supporting evidence shows that Mth1 inhibits protein kinase A‐dependent phosphorylation of Rgt1 that impairs the ability of Rgt1 to interact with Ssn6‐Tup1. Rgt1 can bind DNA in the absence of Ssn6‐Tup1 but fails to inhibit transcription, suggesting that dissociation of Rgt1 from Ssn6‐Tup1, not from DNA, is necessary and sufficient for the expression of its target genes. Taken together, our findings show that Mth1 is a transcription corepressor that facilitates the recruitment of Ssn6‐Tup1 to Rgt1 and reveals a new layer of complexity of eukaryotic gene regulation.