Cyclin K functions as a CDK9 regulatory subunit and participates in RNA polymerase II transcription

Important progress in the understanding of elongation control by RNA polyme rase II (RNAPII) has come from the recent identification of the positive tr anscription elongation factor b (P-TEFb) and the demonstration that this fa ctor is a protein kinase that phosphorylates the carboxyl-terminal domain ( CTD) of the RNAPII largest subunit, The P-TEFb complex isolated from mammal ian cells contains a catalytic subunit (CDK9), a cyclin subunit (cyclin T1 or cyclin T2), and additional, yet unidentified, polypeptides of unknown fu nction, To identify additional factors involved in P-TEFb function we perfo rmed a yeast two-hybrid screen using CDK9 as bait and found that cyclin K i nteracts with CDK9 in vivo, Biochemical analyses indicate that cyclin K fun ctions as a regulatory subunit of CDK9, The CDK9-cyclin K complex phosphory lated the CTD of RNAPII and functionally substituted for P-TEFb comprised o f CDK9 and cyclin T in in vitro transcription reactions.