Functional dissection of the yeast Cyc8–Tupl transcriptional co-repressor complex

Abstract

DNA-BINDING represser proteins mediate regulation of yeast genes by cell type (Mcml/α2 and al/α2), glucose (Migl) and oxygen (Roxl) (refs 1–4 respectively). An unusual feature of all these regulatory pathways is that transcriptional repression requires two physically associated proteins5 that do not bind DNA Cyc8(Ssn6) and Tupl. The Cyc8–Tupl complex has been proposed to be a co-repressor that is recruited to target promoters by pathway-specific DNA-binding proteins6, but the specific functions of the individual proteins are unknown. Here we show that when it is bound upstream of a functional promoter through the Lex A DNA-binding domain, Tupl represses transcription in the absence of Cyc8. Deletion analysis indicates that Tupl contains at least two non-overlapping transcriptional repression regions with minimal primary sequence similarity, and a separable Cyc8-interaction domain. These Tupl domains, which do not include the β-transducin motifs7, are necessary and partially sufficient for Tupl function. We suggest that Tupl performs the repression function of the Cyc8–Tupl co-repressor complex, and that Cyc8 serves as a link with the pathway-specific DNA-binding proteins.