Cyclin C/CDK8 is a novel CTD kinase associated with RNA polymerase II.

A number of cyclin/kinase complexes have been identified in mammalian cells that are essential for controlled cell proliferation. Cyclin C was isolated by virtue of its ability to rescue the triple CLN mutation in yeast; however, until now its function has remained unclear. Cyclin C associates with a novel cyclin dependent kinase, CDK8, and we demonstrate that this complex is associated with kinase activity towards the carboxy-terminal domain (CTD) of RNA polymerase II. We have identified at least two distinct cyclin C/CDK8 containing complexes within the cell, a larger complex over 500 kD in size, that also contains the largest subunit of RNA polymerase II, and a smaller 170 kD species. Both of these cyclin C complexes retain potent CTD kinase activity. We further demonstrate that the cyclin C/CDK8 complex associates with the large subunit of RNA polymerase II in vivo, implicating a potential role for cyclin C/CDK8 in regulating its activities.