| Summary: | The production of eukaryotic mRNA requires transcription by RNA polymerase (pol) II and co-transcriptional processing, including capping, splicing, and cleavage and polyadenylation (CPA). Pol II can positively affect co-transcriptional processing through interaction of factors with its carboxyl terminal domain (CTD), comprising 52 repeats of the heptapeptide Tyr1-Ser2-Pro3-Thr4-Ser5-Pro6-Ser7. Small molecule inhibitors of the splicing factor SF3B1 cause loss of the transcription elongation factor P-TEFb from protein-coding gene templates and major transcription defects, indicating that splicing can, in turn, positively affect transcription. To understand better the relationship between pre-mRNA splicing and pol II transcription, we have investigated the effect of two other splicing inhibitors, Madrasin and Isoginkgetin, on transcription. We found that Madrasin rapidly inhibits pre-mRNA splicing, whereas Isoginkgetin affects transcription before any detectable effect on pre-mRNA splicing. Interestingly, we found that both of these small molecules promote general downregulation of transcription and global transcriptional readthrough, including on intronless and histone genes. Both small molecules affect the association of the mRNA CPA complex with chromatin, likely explaining the transcription termination defect. However, splicing inhibition is not necessarily associated with transcriptional readthrough as small molecule inhibitors of SF3B1 or knockdown of splicing factors do not cause a global transcription termination defect.
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