Fig. 3.

CDK9 regulates transcription across the polyA site. During elongation, CDK9 phosphorylation inactivates PP4 and PP1 phosphatase activities, thus maintaining high levels of Spt5 phosphorylation. Once RNAPII transcribes through the polyA (pA) site, Ser2P recruits the cleavage and polyadenylation machinery (CPA), which is critical for proper termination. Concomittantly, CDK9 activity drops and PP1 becomes active, leading to dephosphorylation of SPT5. Unphosphorylated Spt5 acts as a brake for RNAPII that becomes a preferential substrate for CDK9-activated Xrn2 exonuclease. PP: protein phosphatase. CPA: cleavage and polyadenylation factors