BtBs seminar - Unresolved questions on co-transcriptional splicing

Maria Carmo-Fonseca, Instituto de Medicina Molecular João Lobo Antunes Faculdade de Medicina da Universidade de Lisboa

Maria Carmo-Fonseca Lab

Splicing of pre-mRNAs is a fundamental process in organismal physiology and its misregulation is a hallmark of ageing and many human diseases. Investigating the principles underlying splicing control is paramount for understanding how gene expression intricately shapes human health, and paving the way for the development of innovative RNA targeted treatments. Although the molecular mechanisms of the splicing reactions have been extensively characterized, the principles governing splicing regulation remain elusive. A classical model posits that for short introns, the 5’ and 3’ splice sites are recognized directly by ‘intron definition’. For longer introns, spliceosome assembly starts with the recognition of the downstream exon, and at a later stage a cross-intron complex is formed (‘exon definition’). However, these models had not been directly tested in vivo. In our lab, we have been studying the dynamics of pre-mRNA splicing at a genome-wide scale by purifying RNA polymerase II (Pol II) complexes from the chromatin and sequencing the attached nascent RNAs. Our work in early Drosophila development showed that intron definition can be observed at all intron sizes, arguing against a simple “binary” model, where splicing would systematically switch to exon definition at a particular intron size. Rather, our results are consistent with an alternative model that postulates that previously synthesized exons are tethered to the transcription complex, positioning the splice donor site in close proximity to splice site of the subsequently transcribed exon as it emerges from the RNA exit channel of RNA Pol II. This model implies that splicing is intimately interwoven with transcription. We also performed long-read sequencing of nascent RNA and observed high frequency of all-spliced transcripts, consistent with the view that splicing is rapid enough to match the rate of transcription. However, a significant fraction of nascent transcripts remained unspliced and failed to be cleaved at the polyA site. What is the fate of unspliced pre-mRNAs detected in association with elongating Pol II? This unresolved question in co-transcriptional splicing will be discussed.

Host: Silvia Barabino

The seminar is OPEN to everyone.

Webex connection NOT available

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