The <i>Medicago</i> CDKC;1‐CYCLINT;1 kinase complex phosphorylates the carboxy‐terminal domain of RNA polymerase II and promotes transcription

Abstract

Summary The Ms;CDKC;1 kinase is structurally similar to those cyclin‐dependent kinases (CDKs) that are not involved directly in cell cycle regulation. The presence of a PITAIRE motif in Ms;CDKC;1 suggests that it interacts with cyclins different from known PSTAIRE/PPTALRE kinase regulatory subunits. Here we demonstrate that a Medicago CYCLINT (CYCT) protein is a specific interactor of Ms;CDKC;1 and the interaction between these two proteins gives rise to an active kinase complex that localizes to the nucleus and phosphorylates the carboxy‐terminal YSPTSPS heptapeptide repeat domain (CTD) of the largest subunit of RNA polymerase II in vitro . Mutation of Ser to Ala at position 5 within the heptapeptide repeat abolishes substrate phosphorylation by the Ms;CDKC;1 kinase complex. Furthermore, our data show that addition of the Medicago CDKC;1‐CYCT;1 heterodimer completely restored the transcriptional activity of a HeLa nuclear extract depleted of endogeneous CDK9 kinase complexes. Together, these results indicate that the Medicago CDKC;1‐CYCT;1 complex is a positive regulator of transcription in plants and has a role similar to the CDK9/cyclin T complex of human positive transcription elongation factor P‐TEFb.