Phosphorylation of CTD of a subunit of Pol II

RNA polymerose II is found in a non-phosphorylated form (called IIA) at the time of binding to the promoter. It gets altered into a phosphorylated form (called IIO), before it uncouples from preinitiation complex to take part in the onset and elongation of transcription (Fig. 32.18).

Extensive phosphorylation takes place in the carboxy-terminal domain (CTD) 'tail' of the largest subunit of RNA polymerase II. TFIIH has been shown to be the CTD kinase responsible for phosphorylation of CTD. The kinase is held in check and its activity is increased with the assembly of pre-initiation complex, and with the addition of TFIIE. TFIIH actually consists of five subunits and carries out activities other than phosphorylation of Pol II. In particular, the interaction of TFIIH, TFIIE and TFIIJ may be important for transcription.