Different effects of pre-mRNA splicing inhibitors on RNA polymerase II transcription

The production of a mature mRNA in eukaryotes requires both transcription by RNA polymerase (pol) II and co-transcriptional processes, including mRNA capping, splicing, and cleavage and polyadenylation. The pol II complex serves as a hub to coordinate transcription and co-transcriptional processes, especially via the carboxyl terminal domain (CTD) of the large subunit of pol II that is composed in human of 52 repeats of the heptapeptide Tyr1-Ser2-Pro3-Thr4-Ser5-Pro6-Ser7 and that can be modified by several post-translational modifications. To understand better the effects of pre-mRNA splicing on pol II transcription, we investigated the transcriptional effects of two small molecule inhibitors of splicing: Madrasin and Isoginkgetin. We found that while Madrasin can quickly inhibit pre-mRNA splicing, it is not the case of Isoginkgetin, which affects transcription before any visible effect on pre-mRNA splicing. Interestingly, we found that these two small molecules promote a global transcriptional readthrough, including intronless protein-coding genes and histone genes, which is not the case of the splicing inhibitors targeting the splicing factor SF3B1. mNET-seq to determine the effects of the pre-mRNA splicing inhibitors Madrasin (30 minutes, 90 μM) and Pladienolide B (30 minutes, 1 μM), and of the CDK9 inhibitor DRB (15 minutes, 100 μM) with DMSO as control, on nascent transcription (total RNA polymerase II) and pol II Tyr1, Ser2, Thr4, Ser5, and Ser7 phosphorylation.