Phosphorylation of the RNA polymerase II carboxy-terminal domain by the Bur1 cyclin-dependent kinase

BUR1, which was previously identified by a selection for mutations that hav e general effects on transcription in Saccharomyces cerevisiae, encodes a c yclin-dependent kinase that is essential for viability, but none of its sub strates have been identified to date. Using an unbiased biochemical approac h, we have identified the carboxy-terminal domain (CTD) of Rpb1, the larges t subunit of RNA polymerase II, as a Bur1 substrate. Phosphorylation of Rpb 1 by Bur1 is likely to be physiologically relevant, since burl mutations in teract genetically with rpb1 CTD truncations and with mutations in other ge nes involved in CTD function. Several genetic interactions are presented, i mplying a role for Burl during transcriptional elongation. These results id entify Burl as a fourth S. cerevisiae CTD kinase and provide striking funct ional similarities between Bur1 and metazoan P-TEFb.