The U1 antisense morpholino oligonucleotide (AMO) disrupts U1 snRNP structure to promote intronic PCPA modification of pre-mRNAs.

Functional depletion of the U1 small nuclear ribonucleoprotein (snRNP) with a 25 nt U1 AMO (antisense morpholino oligonucleotide) may lead to intronic premature cleavage and polyadenylation (PCPA) of thousands of genes, a phenomenon known as U1 snRNP telescripting; however, the underlying mechanism remains elusive. In this study, we demonstrated that U1 AMO could disrupt U1 snRNP structure both in vitro and in vivo, thereby affecting the U1 snRNP/RNAP polymerase II (RNAPII) interaction. By performing ChIP-seq for phosphorylation of Ser2 (Ser2P) and Ser5 (Ser5P) of the C-terminal domain (CTD) of RPB1, the largest subunit of RNAPII, we showed that transcription elongation was disturbed upon U1 AMO treatment, with a particular high Ser2P signal at intronic cryptic polyadenylation sites (PASs). In addition, we showed that core 3'processing factors CPSF/CstF are involved in the processing of intronic cryptic PAS. Their recruitment accumulated toward cryptic PASs upon U1 AMO treatment, as indicated by ChIP-seq and iCLIP-seq analysis. Conclusively, our data suggest that disruption of U1 snRNP structure mediated by U1 AMO provides a key for understanding the U1 telescripting mechanism.