Histone H2B Monoubiquitylation Regulates Elongation-to-Termination Transition in RNA Polymerase II Transcription

RNA Polymerase II (Pol II)-dependent transcription is meticulously regulated during initiation, elongation, termination, and two pivotal transitions: from initiation to elongation and from elongation to termination. The successful transition from elongation to termination is vital for accurate transcription termination. While elongation and termination have been extensively studied, the regulatory mechanisms governing the transition between these stages are not fully understood. Here we report that, in fission yeast Schizosaccharomyces pombe (S. pombe) and human cells, Cdk9 and its cyclin partner (Cyclin T in humans, and Pch1 in S. pombe) disengage from the elongation machinery near gene 3'-ends, as Pol II crosses the cleavage and polyadenylation signal (CPS). Concurrently, chromatin immunoprecipitation (ChIP) data shows an increased association of Dis2 (the PP1 ortholog in S. pombe) beyond the CPS. Additionally, ChIP-seq indicates that histone H2B monoubiquitylation (H2Bub1) is pivotal for Cdk9 occupancy but serves differential roles along the gene. H2Bub1 facilitates Cdk9 chromatin recruitment during elongation but prompts Cdk9 dissociation from the elongation complex as Pol II traverses the CPS. Furthermore, in S. pombe, disruptions of H2Bub1 in htb1-K119R and sustained levels in ubp8D inversely affect the chromatin occupancy of Pol II, Pol II CTD phosphorylation within the heptad repeats, Spt5, phosphorylated Spt5 (pSpt5), Dis2, components of the mRNA 3’-end processing complex (Pfs2 and Pla1), and termination factors Rhn1 (Rtt103) and Pcf1 around the CPS—diminishing with H2Bub1 loss and increasing with H2Bub1 stabilization. In essence, our data indicates that the H2Bub1-mediated eviction of Cdk9 as Pol II navigates the CPS precipitates a rise in PP1 binding, which, in turn, dephosphorylates pSpt5, slowing Pol II to facilitate efficient termination. The unique modulation of Cdk9 occupancy by H2Bub1 during transcription progression presents a compelling, novel concept requiring further exploration.