CAR-SPLASH identifies nascent pre-mRNA structures implicated in kinetic coupling and alternative splicing [smallRNA-Seq]
Transcription and co-transcriptional pre-mRNA splicing are kinetically coupled as shown by the widespread effects of transcription speed on alternative splicing. The molecular basis for kinetic coupling is incompletely understood, but one potential mechanism is through elongation rate dependent alternative folding pathways of the nascent pre-mRNA. To search for RNA structures in nascent pre-mRNA, we modified SPLASH (1) (Sequencing of Psoralen Crosslinked, Ligated And Selected Hybrids) for use with Chromatin Associated RNA. We applied this new method called CAR-SPLASH to cells expressing WT and slow mutant pol II and identified >3000 intragenic RNA structures of which  >400 are proximal to splice sites. ASO disruption of several structures identified by CAR-SPLASH that sequester splice sites changed alternative splicing outcomes in vivo. We identified three examples of novel regulatory elements we designate “RNA kinetic switches”. ASO disruption of these structures modified alternative splicing of NISCH Exon 18 GAK Exon 7 and MEGF8 Exon 24 in a way that depended on the rate of transcription elongation. These results demonstrate that individual RNA structures can mediate kinetic coupling between transcription and pre-mRNA splicing and that nascent RNA structures can serve as targets for splice modifying ASO's. Overall design: HEK293 Flp-in cells expressing WT or slow pol II mutant (Rpb1 R749H) a-amanitin resistant pol II were cross-linked with a biotinylated Psoralen analogue. Cross-linked chromatin associated RNA was purified on streptavidin beads and libraries made of proximity ligated RNA fragments.