Conserved protein Seb1 that interacts with RNA polymerase II and RNA is a bona fide transcription elongation factor

Maturation of protein-coding precursor messenger RNA (pre-mRNA) is closely linked to RNA polymerase II (Pol II) transcription. However, the mechanistic understanding of how RNA processing is coordinated with transcription is incomplete. Conserved proteins interacting with the C-terminal domain of the largest catalytic subunits of Pol II and nascent RNA (CID-RRM factors) were demonstrated to play a role in mRNA 3-end processing and termination of Pol II transcription. Here, we employ a fully reconstituted system to demonstrate that fission yeast CID-RRM factor Seb1 acts as a bona fide elongation factor in vitro. Our analyses show that Seb1 exhibits context-dependent regulation of Pol II pausing, capable of either promoting or inhibiting pause site entry. We propose that CID-RRM factors coordinate Pol II transcription and RNA 3-end processing by modulating the rate of Pol II transcription.