The competition between splicing and 3′ processing shapes the human transcriptome

Eukaryotic pre-mRNA processing steps, including splicing and 3′ processing, are tightly coordinated, but the underlying mechanisms remain poorly understood. Previous studies proposed that the splicing factor U1 snRNP inhibits 3′ processing at intronic polyadenylation (IPA) sites through a splicing-independent mechanism, called telescripting. However, we found that global or gene-specific perturbation of splicing by targeting multiple splicing factors, including U1 snRNP, U2 snRNP, U2AF, and SF3b led to activation of 3′ processing at IPA sites. Inhibiting different splicing factors activated overlapping and distinct IPA sites and such specificity was determined, at least in part, by alterations in RNA polymerase II elongation and termination. Conversely, we showed that blocking pre-mRNA 3′ processing promoted splicing globally. These results strongly suggest that splicing and 3′ processing are competing processes that shape the transcriptome. Finally, as splicing inhibition-induced shifts to IPA site usage can lead to gene inactivation, including tumor suppressor genes, the use of general splicing inhibitors to treat human diseases may pose a significant risk.