Figure 5.

Transcription coupled events. Once the PIC has formed, and Ser7 and Ser5 phosphorylation of the RNAPII CTD by CDK7 (as part of TFIIH) allows initiation and promoter escape to occur. Shortly after transcription of the first 25–30 nt the 5’end of the nascent RNA is capped by the capping enzymes Mce1 and Hcm1. This process occurs more efficiently when RNAPII CTD is phosphorylated at Ser5 residues. Elongation is induced by phosphorylation events by CDK9, including CTD Ser2 phosphorylation. pSer2 is increased and RNAPII moves down the gene body by CDK12/13. This enhances recruitment of splice factors and production of fully spliced mRNAs. Finally, the recruitment of RNA cleavage factors are dependent on pSer2. The generation of cleaved 3’ends occurs 10–35 nt downstream of the polyA consensus sequence and the enzyme PAP adds 200–300 A residues to 3’OH group. Once the mRNA is released from RNAPII and CTD phosphorylation is removed by phosphatases, it is free to participate in the formation of the PIC to initiate new rounds of transcription.