Cyclin-dependent Kinase Inhibitor p16INK4A Inhibits Phosphorylation of RNA Polymerase II by General Transcription Factor TFIIH

The cell cycle is regulated by various protein kinases, including cyclin-dependent kinases (CDKs). D-type CDKs, CDK4, and CDK6, phosphorylate retinoblastoma protein and are believed to regulate through the G1 phase of the cell cycle. CDK inhibitor p16INK4A has been characterized as binding CDK4 and CDK6 and as inhibiting phosphorylation of retinoblastoma protein by these CDKs. Thus p16INK4A is implicated in regulating the cell cycle at the G1 phase. The largest subunit of RNA polymerase II (pol II) contains an essential C-terminal domain (CTD). General transcription factor TFIIH, which contains CDK7, phosphorylates the CTD in vitro. The CTD phosphorylation is shown to be involved in transcriptional regulation in vivo and in vitro. Phosphorylation of RNA pol II CTD by TFIIH is thought to play an important role in transcriptional regulation. Here we report that p16INK4Aassociates with RNA pol II CTD and TFIIH. p16INK4Ainhibited the CTD phosphorylation by TFIIH. These findings suggest that p16INK4A may regulate transcription via CTD phosphorylation in the cell cycle.