14-3-3 proteins, known as Bmh in yeast, are ubiquitous, highly conserved proteins that function as adaptors in signal transduction pathways by binding to phosphorylated proteins to activate, inactivate, or sequester their substrates. Bmh proteins have an important role in glucose repression by binding to Reg1, the regulatory subunit of Glc7, a protein phosphatase that inactivates the AMP-activated protein kinase Snf1. We describe here another role for Bmh in glucose repression. We show that Bmh binds to the Snf1-dependent transcription factor Adr1 and inhibits its transcriptional activity. Bmh binds within the regulatory domain of Adr1 between amino acids 215 and 260, the location of mutant ADR1(c) alleles that deregulate Adr1 activity. This provides the first explanation for the phenotype resulting from these mutations. Bmh inhibits Gal4-Adr1 fusion protein activity by binding to the Ser230 region and blocking the function of a nearby cryptic activating region. ADR1(c) alleles, or the inactivation of Bmh, relieve the inhibition and Snf1 dependence of this activating region, indicating that the phosphorylation of Ser230 and Bmh are important for the inactivation of Gal4-Adr1. The Bmh binding domain is conserved in orthologs of Adr1, suggesting that it acquired an important biological function before the whole-genome duplication of the ancestor of S. cerevisiae.