Cdk1 triggers association of RNA polymerase to cell cycle promoters only after recruitment of the mediator by SBF

Activation of HO in yeast involves recruitment of transcription factors in two waves. The first is triggered by inactivation of Cdk1 at the end of mit osis, which promotes import into the nucleus of the Swi5 transcription fact or. Swi5 recruits the Swi/Snf chromatin-remodeling complex, which then faci litates recruitment of the SAGA histone acetylase, which in turn permits th e binding of the SBF transcription factor. We show here that SBF then recru its the SRB/mediator complex and that this process occurs in the absence of Cdk1 activity. The second wave is triggered by reactivation of Cdk1, which leads to recruitment of PoIII, TFIIB, and TFIIH. RNA polymerase is, theref ore, recruited to HO in two steps and not as a holoenzyme. A similar sequen ce of events occurs at other SBF-regulated promoters, such as CLN1, CLN2, a nd PCL1.