Five of the cyclin-dependent kinases (CDKs: 7, 8, 9, 12, and 13) are involved in the phosphorylation of serine residues in the C-terminal domain of RNA polymerase II, the enzyme that is responsible for the transcription of protein-coding genes into mRNA in eukaryotes. Phosphorylation of RNA polymerase II at Ser5 is essential for transcriptional initiation, and phosphorylation of Ser 2 contributes to transcriptional elongation and termination. All five of the C-terminal domain kinases can phosphorylate Ser5, but only CDK9, CDK12, and CDK13 can phosphorylate at Ser2 [1,3-4].

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The development of CDK inhibitors as anticancer drugs is reviewed in [5], with detailed content covering CDK4 and CDK6 inhibitors that are under clinical evaluation. Data produced by Jorda et al. (2018) highlights the caution that must be used when deploying commercially available CDK inhibitors as pharmacological probes [2], as most of them are more promiscuous in their selectivity than indicated. To make their findings easily accessible the Jorda data is hosted on the cyclin-dependent kinase inhibitor database (CDKiDB).

1. Bowman EA, Kelly WG. (2014) RNA polymerase II transcription elongation and Pol II CTD Ser2 phosphorylation: A tail of two kinases. Nucleus, 5 (3): 224-36. [PMID:24879308]

2. Jorda R, Hendrychová D, Voller J, Řezníčková E, Gucký T, Kryštof V. (2018) How Selective Are Pharmacological Inhibitors of Cell-Cycle-Regulating Cyclin-Dependent Kinases?. J Med Chem, 61 (20): 9105-9120. [PMID:30234987]

3. Kohoutek J, Blazek D. (2012) Cyclin K goes with Cdk12 and Cdk13. Cell Div, 7: 12. [PMID:22512864]

4. Liang K, Gao X, Gilmore JM, Florens L, Washburn MP, Smith E, Shilatifard A. (2015) Characterization of human cyclin-dependent kinase 12 (CDK12) and CDK13 complexes in C-terminal domain phosphorylation, gene transcription, and RNA processing. Mol Cell Biol, 35 (6): 928-38. [PMID:25561469]

5. Sánchez-Martínez C, Gelbert LM, Lallena MJ, de Dios A. (2015) Cyclin dependent kinase (CDK) inhibitors as anticancer drugs. Bioorg Med Chem Lett, 25 (17): 3420-35. [PMID:26115571]