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.