Activation of MyoD-dependent transcription by cdk9/cyclin T2

Myogenic transcription is repressed in myoblasts by serum-activated cyclin-dependent kinases, such as CDK2 and CDK4. Serum withdrawal promotes muscle-specific gene expression at least in part by down-regulating the activity of these cdks. Unlike the other cdks, CDK9 is not serum- or cell cycle-regulated and is instead involved in the regulation of transcriptional elongation by phosphorylating the carboxyl-terminal domain (CTD) of RNA polymerase II. While ectopic expression of CDK2 together with its regulatory subunits (cyclins E and A) inhibits myogenic transcription, overproduction of CDK9 and its associated cyclin (cyclin T2a) strengthens MyoD-dependent transcription and stimulates myogenic differentiation in both MyoD-converted fibroblasts and C2C12 muscle cells. Conversely, inhibition of CDK9 activity by a dominant negative form (cdk9-dn) represses the myogenic program. CDK9, cyclinT2 and MyoD can be detected in a multimeric complex in C2C12 cells, with the minimal cdk9-binding region of MyoD mapping within 101-161 aa of the bHLH region. Finally, CDK9 can phosphorylate MyoD in vitro, suggesting the possibility that CDK9/cycT2a regulation of muscle differentiation includes the direct enzymatic activity of the kinase on MyoD.