The cyclin in the RNA polymerase holoenzyme is a target for the transcriptional repressor Tup1p in Saccharomyces cerevisiae.

The general transcriptional repressor Tup1p requires the cyclin/cyclin-dependent kinase pair Srb11p/Srb10p in the holoenzyme of transcription. We used the split-ubiquitin system to demonstrate that Tup1p interacts with Srb11p in vivo. We confirmed our observation in vitro with the help of purified proteins, and we compared the de-repression effect of deleting TUP1, SRB10, and SRB11 on different promoters. We propose that Tup1p targets the cyclin Srb11p to affect the cyclin-dependent kinase Srb10p.