Figure 2.

Transcription and its associated CDK/cyclin complexes. RNA Pol II forms part of the pre-initiation complex that starts gene transcription in eukaryotes. This pre-initiation complex is inhibited by the Mediator complex, of which CDK8 and cyclin C are part. The Mediator complex represses transcription by phosphorylating the C-terminal domain (CTD) of RNA Pol II to prevent its recruitment to the promoter and by phosphorylating cyclin H. Cyclin H in complex with CDK7 forms part of the transcription factor complex TFIIH, which in turn phosphorylates the CTD of RNA Pol II, triggering the transition from transcription initiation to mRNA elongation. TFIIH phosphorylates cyclin T as well. The CDK9/cyclin T complex forms part of the positive transcription elongation factor b (pTEFb), which phosphorylates the RNA Pol II CTD, thus promoting the extension of the pre-mRNA.