DROSOPHILA CDK8, A KINASE PARTNER OF CYCLIN-C THAT INTERACTS WITH THELARGE SUBUNIT OF RNA-POLYMERASE-II

A number of cyclins have been described, most of which act together wi th their catalytic partners, the cyclin-dependent kinases (Cdks), to r egulate events in the eukaryotic cell cycle. Cyclin C was originally i dentified by a genetic screen for human and Drosophila cDNAs that comp lement a triple knock-out of the CLN genes in Saccharomyces cerevisiae . Unlike other cyclins identified in this complementation screen, ther e has been no evidence that cyclin C has a cell-cycle role in the cogn ate organism. Here we report that cyclin C is a nuclear protein presen t in a multiprotein complex. It interacts both in vitro and in vivo wi th Cdk8, a novel protein-kinase of the Cdk family, structurally relate d to the yeast Srb10 kinase. We also show that Cdk8 can interact in vi vo with the large subunit of RNA polymerase II and that a kinase activ ity that phosphorylates the RNA polymerase II large subunit is present in Cdk8 immunoprecipitates. Based on these observations and sequence similarity to the kinase/cyclin pair Srb10/Srb11 in S. cerevisiae, we suggest that cyclin C and Cdk8 control RNA polymerase II function.