Regulation of the Bur1 protein kinase
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Cyclin-dependent kinases (CDKs) and their cyclins were originally identified as key regulators of cell cycle progression, but subsequent studies found that some CDKs also perform very important roles in transcription regulation. BUR1 and BUR2 were previously identified in our lab by a genetic selection for mutations that increase transcription from a basal promoter in vivo. BUR1 encodes a kinase with sequence similarity to members of the CDK family. Utilizing both genetic and biochemical approaches, we found that BUR2 encodes a cyclin for Bur1, revealing Bur1-Bur2 as a novel CDK-cyclin complex in Saccharomyces cerevisiae. To more precisely define the specific function of Bur1, we have attempted to identify substrates for the Bur1-Bur2 complex. Genetic and biochemical evidence shows that Bur1 affects transcription by phosphorylating the carboxy-terminal domain (CTD) of Rpb1, the largest subunit of RNA polymerase II.;To further investigate the regulation of Bur1 in vivo, we searched for high copy number suppressors of a bur1 temperature-sensitive mutation, identifying a single gene, CAK1. Cak1 is known to phosphorylate two other CDKs at a threonine within their conserved T-loop activation domains: Cdc28, which controls the yeast cell cycle, and Kin28, a subunit of the general transcription factor TFIIH. Additional evidence demonstrates that Cak1 is a physiological regulator of Bur1, and that Cak1 stimulates the Bur1-Bur2 kinase activity against the Rpb1 CTD.;Identification of the Bur1-Bur2 complex as a novel CDK-cyclin complex involved in transcription regulation, potentially through phosphorylating the Rpb1 CTD, introduced another level of complexity into the central puzzle: how does phosphorylation of the CTD coordinate multiple interwoven transcriptional events? Continued studies on BUR1 and BUR2 are certain to yield interesting insights into their specific functions during the transcription cycle and their potential overlap with other CDK-cyclin complexes that have general roles in transcription.