Phosphorylation initiation-elongation transition

Fig. 1.32. Phosphorylation of the C-terminal domain of RNA polymerase II and the beginning of transcription. The transition from the initiation complex to actual begin of transcription is regulated via phosphorylation of the C-terminal domain (CTD) of RNA polymerase II. In the above model it is assumed that initially a complex is formed between TFIID and a holoenzyme of RNA polymerase consisting of RNA polymerase II and associated factors (mediators, SRB proteins) and the basal transcription factors. Phosphorylation of the C-terminal domain effects the dissociation of the RNA polymerase from the initation complex and the transition to the elongation phase. A protein kinase, which is part of TFIIH, is responsible for the phosphorylation. The nature of the signal that induces phosphorylation of RNA polymerase II remains unknown. SRB suppressor of RNA polymerase B. After Koleske and Young (1995).

An example for how protein phosphorylation can influence the transcription process is the transition from the initiation to the elongation process for RNA polymerase II (see 1.4.2.4). 

Figure 29.34 The CTD Coupling transcription to pre-mRNA processing. The transcription factor TFIIH phosphorylates the car boxy I-terminal domain (CTD) of RNA polymerase II, signaling the transition from transcription initiation to elongation. The phosphorylated CfD binds factors required for pre-mRNA capping, splicing, and polyadenyiation. These proteins are brought in close proximity lo their sites of action on the nascent pre-mRNA as it is transcribed during elongation.

TFIIH is required for critical, early steps of transcription, including the transition from initiation to elongation. In addition to its function in phosphorylating the CTD, TFIIH opens the DNA template by its ATP-dependent helicase activities located on the XPB and XPD proteins. Electron microscope studies have revealed a ring-like structure of TFIIH, with a hole whose size is able to accommodate a double-stranded DNA (Chang and Kornberg, 2000). 

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