Cyclin C/CDK8 and cyclin H/CDK7/p36 are biochemically distinct CTD kinases

Oncogene. 1999 Jan 28;18(4):1093-102. doi: 10.1038/sj.onc.1202399.


Phosphorylation of the carboxyl-terminal domain (CTD) of RNA polymerase II is important for basal transcriptional processes in vivo and for cell viability. Several kinases, including certain cyclin-dependent kinases, can phosphorylate this substrate in vitro. It has been proposed that differential CTD phosphorylation by different kinases may regulate distinct transcriptional processes. We have found that two of these kinases, cyclin C/CDK8 and cyclin H/CDK7/p36, can specifically phosphorylate distinct residues in recombinant CTD substrates. This difference in specificity may be largely due to their varying ability to phosphorylate lysine-substituted heptapeptide repeats within the CTD, since they phosphorylate the same residue in CTD consensus heptapeptide repeats. Furthermore, this substrate specificity is reflected in vivo where cyclin C/ CDK8 and cyclin H/CDK7/p36 can differentially phosphorylate an endogenous RNA polymerase II substrate. Several small-molecule kinase inhibitors have different specificities for these related kinases, indicating that these enzymes have diverse active-site conformations. These results suggest that cyclin C/CDK8 and cyclin H/CDK7/p36 are physically distinct enzymes that may have unique roles in transcriptional regulation mediated by their phosphorylation of specific sites on RNA polymerase II.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Cyclin C
  • Cyclin H
  • Cyclin-Dependent Kinase 8
  • Cyclin-Dependent Kinases*
  • Cyclins / metabolism*
  • Peptide Fragments / metabolism*
  • Phosphorylation
  • Protein-Serine-Threonine Kinases / metabolism*
  • RNA Polymerase II / chemistry
  • RNA Polymerase II / metabolism*
  • Substrate Specificity
  • Transcriptional Activation


  • CCNC protein, human
  • CCNH protein, human
  • Cyclin C
  • Cyclin H
  • Cyclins
  • Peptide Fragments
  • Protein-Serine-Threonine Kinases
  • CDK8 protein, human
  • Cyclin-Dependent Kinase 8
  • Cyclin-Dependent Kinases
  • cyclin-dependent kinase-activating kinase
  • RNA Polymerase II