Meta-Biol

• Pathways

CDK12, in complex with CCNK (cyclin K), phosphorylates heptapeptide repeats in the C-terminal domain (CTD) of the RNA polymerase II (RNA Pol II) subunit POLR2A. CDK12 may require phosphorylation of its threonine residue T893 to achieve full catalytic activity, but the activating kinase is not known. CDK12-mediated phosphorylation of the CTD of POLR2A occurs after the heptapeptide repeats in the CTD of POLR2A undergo phosphorylation by the CDK9-containing P-TEFb complex. It is unclear whether CDK12 acts on the second serine or the fifth serine or both in the YSPTSPS repeats. The mammalian POLR2A contains 52 heptapeptide repeats that start at amino acid position 1615. The exact localization of CDK9 and CDK12 target sites relative to the full-length POLR2A is not known. CDK12-mediated phosphorylation of the pre-phosphorylated RNA Pol II complex is important for the transcription of a group of genes with long and complex structures, involved in DNA repair (Blazek et al. 2011, Cheng et al. 2012, Bosken et al. 2014, Bartkowiak and Greenleaf 2015, Liang et al. 2015).