FUNCTIONAL DISSECTION OF THE YEAST CYC8-TUP1 TRANSCRIPTIONAL CO-REPRESSOR COMPLEX

DNA-BINDING repressor proteins mediate regulation of yeast genes by ce ll type (Mcm1/alpha 2 and a1/alpha 2), glucose (Mig1) and oxygen (Rox1 ) (refs 1-4 respectively). An unusual feature of all these regulatory pathways is that transcriptional repression requires two physically as sociated proteins(5) that do not bind DNA Cyc8(Ssn6) and Tup1. The Cyc 8-Tup1 complex has been proposed to be a corepressor that is recruited to target promoters by pathway-specific DNA-binding proteins(6), 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 LexA DNA-binding domain, Tup1 represses transcription in the absen ce of Cyc8. Deletion analysis indicates that Tup1 contains at least tw o non-overlapping transcriptional repression regions with minimal prim ary sequence similarity, and a separable Cyc8-interaction domain. Thes e Tup1 domains, which do not include the beta-transducin motifs(7), ar e necessary and partially sufficient for Tup1 function. We suggest tha t Tup1 performs the repression function of the Cyc8-Tup1 co-repressor complex, and that Cyc8 serves as a link with the pathway-specific DNA- binding proteins.