BUR1 and BUR2 encode a divergent cyclin-dependent kinase-cyclin complex important for transcription in vivo

BUR1 and BUR2 were previously identified by a genetic selection for mutatio ns that increase transcription from basal promoters in vivo, BUR1 encoded a putative protein kinase with greatest similarity to members of the cyclin- dependent kinase (CDK) family, although that similarity was not sufficient to classify it as a CDK. It was also not known whether Bur1 activity was cy clin dependent and, if so, which cyclins stimulated Bur1. The molecular clo ning and characterization of BUR2 presented here sheds light on these issue s. Genetic analysis indicates that BUR2 function is intimately related to t hat of BUR1: bur1 and bur2 mutations cause nearly identical spectra of muta nt phenotypes, and overexpression of BUR1 suppresses a bur2 null allele. Bi ochemical analysis has provided a molecular basis for these genetic observa tions. We find that BUR2 encodes a cyclin for the Bur1 protein kinase, base d on the following evidence. First, the BUR2 amino acid sequence reveals si milarity to the cyclins; second, Bur1 and Bur2 coimmunoprecipitate from cru de extracts and interact in the two-hybrid system; and third, BUR2 is requi red for Bur1 kinase activity in vitro. Our combined genetic and biochemical results therefore indicate that Bur1 and Bur2 comprise a divergent CDK-cyc lin complex that has an important functional role during transcription in v ivo.