3′ end formation of pre-mRNA and phosphorylation of Ser2 on the RNA polymerase II CTD are reciprocally coupled in human cells

Abstract

Pre-mRNA 3′ end formation is coupled to transcription via the RNA Pol II C-terminal domain (CTD). However, how transcription and pre-mRNA maturation are coordinated in humans is poorly understood. Here, West and colleagues show that Pol II pausing promotes Ser2p by Cdk12, which recruits CPA factor CstF77 and is required for optimal 3′ end processing. This study delineates a reciprocal relationship between early steps in poly(A) site processing and Pol II Ser2p that ensures efficient pre-mRNA 3′ end formation in human cells.

Abstract

3′ end formation of pre-mRNAs is coupled to their transcription via the C-terminal domain (CTD) of RNA polymerase II (Pol II). Nearly all protein-coding transcripts are matured by cleavage and polyadenylation (CPA), which is frequently misregulated in disease. Understanding how transcription is coordinated with CPA in human cells is therefore very important. We found that the CTD is heavily phosphorylated on Ser2 (Ser2p) at poly(A) (pA) signals coincident with recruitment of the CstF77 CPA factor. Depletion of the Ser2 kinase Cdk12 impairs Ser2p, CstF77 recruitment, and CPA, strongly suggesting that the processes are linked, as they are in budding yeast. Importantly, we additionally show that the high Ser2p signals at the 3′ end depend on pA signal function. Down-regulation of CPA results in the loss of a 3′ Ser2p peak, whereas a new peak is formed when CPA is induced de novo. Finally, high Ser2p signals are generated by Pol II pausing, which is a well-known feature of pA site recognition. Thus, a reciprocal relationship between early steps in pA site processing and Ser2p ensures efficient 3′ end formation.