A number of protein kinases have been implicated in the phosphorylation of the CTD during the transition from transcription initiation to elongation. Among these are the Cdk7 subunit of the GTF IIH (TFIIH) and the Cdk9 subunit of P-TEFb, a factor that regulates transcription elongation. The Cdk7 kinase phosphorylates serine 5 of the CTD heptapeptide motif. And, while the specificity of human P-TEFb remains controversial, a close yeast homologue of its Cdk9 subunit, Ctk1, modifies the serine residue at position 2 of the CTD heptapeptide repeat.
Mandal, S. S., Cho, H., Kim, S., Cabane, K., & Reinberg, D. (2002). FCP1, a phosphatase specific for the heptapeptide repeat of the largest subunit of RNA polymerase II, stimulates transcription elongation. Molecular and cellular biology, 22(21), 7543-7552.
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