Pseudouridylation of 7SK by PUS7 regulates Pol II transcription elongation

Pseudouridine (Ψ) is a widespread RNA modification in various RNA species, including rRNA, tRNA, snRNA and mRNA. Ψ plays a crucial role in RNA metabolism, where it regulates pre-mRNA splicing and affects protein translation. Whether and how Ψ may regulate transcription have not be adequately studied. Here, we report that pseudorudine synthase 7 (PUS7) can mediate pseudouridylation of 7SK small nuclear RNA (snRNA), a regulator of RNA polymerase II (Pol II) promoter-proximal pausing. PUS7 loss leads to hypo-pseudouridylation and facilitates sequestration of the positive transcription elongation factor b (P-TEFb) complex from 7SK. The release of P-TEFb from 7SK increases serine 2 phosphorylation (Ser2P) in the RNA Pol II C-terminal domain and enhances transcription elongation. In colorectal cancer (CRC) cells, the Ψ level of 7SK could be modulated by PUS7, or by site-specifically targeted pseudouridylation through dCas13b-guided system. Hypo-pseudouridylation on 7SK with PUS7 depletion promotes KLF6/DDIT3-mediated cell apoptosis and sensitizes CRC cells to 5-FU. We performed BID-seq, PAR-CLIP, KAS-seq and mRNA-seq to study the role PUS7 plays in transcription process. BID-seq, PAR-CLIP and mRNA-seq showed 7SK is the key substrate of PUS7. KAS-seq indicated that PUS7 regulates transcrition elongation process by tuning the pseudouridylation of 7SK.