THE RNA polymerase II holoenzyme consists of RNA polymerase II, a subset of general transcription factors, and regulatory proteins known as SRB proteins1, 2. The genes encoding SRB proteins were isolated as suppressors of mutations in the RNA polymerase II carboxy-terminal domain (CTD)3, 4. The CTD and SRB proteins have been implicated in the response to transcriptional regulators1–11. We report here the isolation of two new SRB genes, SRB10 and SRB11, which encode kinase- and cyclin-like proteins, respectively. Genetic and biochemical evidence indicates that the SRB 10 and SRB 11 proteins form a kinase–cyclin pair in the holoenzyme. The SRB10/11 kinase is essential for a normal transcriptional response to galactose induction in vivo. Holoenzymes lacking SRB 10/11 kinase function are strikingly deficient in CTD phosphory lation. Although defects in the kinase substantially affect transcription in vivo, purified holoenzymes lacking SRB10/11 kinase function do not show defects in definedin vitro transcription systems, suggesting that the factors necessary to elicit the regulatory role of the SRB10/11 kinase are missing in these systems. These results indicate that the SRB 10/11 kinase is involved in CTD phosphorv lation and suggest that this modification has a role in the response to transcriptional regulators in vivo. © 1995 Nature Published Ltd. All rights reserved.