The PAF1 Complex Regulates Transcriptional Termination and Reinitiation

Chromatin places fundamental physical constraints on transcription (Gamarra and Narlikar, 2021). The PAF1 complex (PAF1C), a hexamer of PAF1, LEO1, CTR9, SKI8, CDC73 and RTF1, plays a critical role in transcription with incompletely understood mechanisms. During transcriptional elongation, PAF1C is one of the positive elongation factors in complex with RNA polymerase II (Pol II) to facilitate elongation through chromatin (Vos et al., 2018), but it is unclear yet if negative elongation factors are needed concurrently to restrain elongation. Here we show that besides decreasing elongation rate, LEO1 knockout in human K562 cells increases transcriptional readthrough and cellular level of C-terminal domain (CTD) phosphorylated Pol II while increases and decreases transcriptional output of several hundred genes, respectively. Mechanistic analyses taking proteomic, functional genomic and biochemical approaches discovered that PAF1C regulates transcriptional termination in part through recruiting PNUTS-PP1γ complex and facilitates Pol II transcriptional re-initiation through recruiting TOX4-PP1α complex. Moreover, Paf1 conditional knockout in mice severely blocks T cell development, increases cellular level of CTD phosphorylated Pol II, mainly decreases Pol II occupancy and transcriptional output in double positive T cells, and importantly, the regulation of re-initiation by PAF1C through TOX4-PP1α complex is conserved between mouse and human. Our results also suggest that PNUTS-PP1γ and TOX4-PP1α bind PAF1C to restrain Pol II elongation through chromatin. These findings not only establish PAF1C as a critical regulator of transcriptional termination and re-initiation besides elongation but also advanced current understanding of elongation.