Phosphorylation Driven Regulatory Switch from Transcription Initiation to Elongation

The RNA polymerase II (pol II) A form contains an unphosphorylated carboxy-terminal domain (CTD) which initiates transcription producing 20-25 nucleotide 5’-triphosphorylated transcripts. The CTD is then phosphorylated to the RNA pol II O form, changing conformation and binding to RNA capping enzyme (CE). Stimulated by the elongation factor SPT5 and the phosphorylated CTD, CE which has RNA triphosphatase and RNA guanylyltransferase activities modifies the exposed end of the nascent transcript to GpppN. Methyl transferase (MT) binds to CE and to the terminal GpppN. Finally, Importin-a (Impa) stimulates N7 methylation of the cap GpppN by MT, and RNA pol II O form switches from the initiation process to elongation. In addition, the pol II CTD also binds splicing factors and polyadenylation factors. Hence, the pol II CTD is central to the cotranscriptional nature of RNA processing (5’ capping, splicing and 3’polyadenylation).

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Adapted from Shatkin, A.J. et al., Nature Structural Biology, 7, 838, (2000) and McCracken, S. et al., Genes & Development 11, 3306, (1997).