ASSOCIATION OF CDK-ACTIVATING KINASE SUBUNITS WITH TRANSCRIPTION FACTOR TFIIH

THE RNA polymerase II large subunit contains an essential carboxy-term inal domain (CTD) believed to be involved in the response to regulator s during transcription initiation(1-10). The CTD is phosphorylated on a portion of RNA polymerase II molecules in vivo(11,12) and it can be phosphorylated by the general transcription factor TFIIH in vitro(13-1 5). A highly purified TFIIH from rat liver has been described(16); thi s, like human and yeast TFIIH, contains associated CTD kinase and heli case activities(13-18). We report here that two polypeptides of the pu rified mammalian TFIIH are the MO15/Cdk7 kinase and cyclin H subunits of the Cdk-activating kinase Cak(19-21), previously identified as a po sitive regulator of Cdc2 and Cdk2. TFIIH and Cak preparations are each capable of phosphorylating recombinant CTD and recombinant Cdk2 prote ins. The presence of Cak in TFIIH indicates that Cak may have roles in transcriptional regulation and in cell-cycle control.