THE RNA polymerase II large subunit contains an essential carboxy-terminal domain (CTD) believed to be involved in the response to regulators during transcription initiation1–10. The CTD is phosphorylated on a portion of RNA polymerase II molecules in in vivo11, 12 and it can be phosphorylated by the general transcription factor TFIIH in vitro13–15. A highly purified TFIIH from rat liver has been described16; this, like human and yeast TFIIH, contains associated CTD kinase and helicase activities13–18. We report here that two polypeptides of the purified mammalian TFIIH are the MO15/Cdk7 kinase and cyclin H subunits of the Cdk-activating kinase Cak19–21, previously identified as a positive regulator of Cdc2 and Cdk2. TFIIH and Cak preparations are each capable of phosphorylating recombinant CTD and recombi-nant Cdk2 proteins. The presence of Cak in TFIIH indicates that Cak may have roles in transcriptional regulation and in cell-cycle control.
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