A unified mechanism for intron and exon definition and back-splicing

Abstract

The molecular mechanisms of exon definition and back-splicing are fundamental unanswered questions in pre-mRNA splicing. Here we report cryoEM structures of the yeast E complex assembled on introns, providing the first view of the earliest event in the splicing cycle that commits pre-mRNAs to splicing. The E complex architecture suggests that the same spliceosome can assemble across an exon, which either remodels to span an intron for canonical linear splicing (typically on short exons) or catalyzes back-splicing generating circRNA (on long exons). The model is supported by our experiments demonstrating that E complex assembled on the yeast EFM5 or HMRA1 middle exon can be chased into circRNA when the exon is sufficiently long. Users may view, print, copy, and download text and data-mine the content in such documents, for the purposes of academic research, subject always to the full Conditions of use:

In-Depth Analysis Reveals Production of Circular RNAs from Non-Coding Sequences

“…In the meantime, evidence has accumulated that circRNAs are evolutionarily conserved and their expression levels vary with the tissue and with the developmental stage, suggesting that circRNAs have regulatory functions [3,4]. Circular RNA are probably a natural byproduct of the splicing process in all eukaryotes [5]. Splicing is the mechanism by which nascent precursor messenger RNA (pre-mRNA) is edited into mature mRNA and which is mediated by a protein-RNA complex known as the spliceosome.…”

“…In contrast to canonical splicing, which joins an upstream (5 ) splice donor site and a downstream (3 ) splice acceptor site, back splicing ligates a downstream splice donor site reversely with an upstream splice acceptor site, resulting in a covalently closed circRNA transcript. Back splicing is a peculiar splicing reaction that generates a class of circRNAs that can be described by identifying the two joined exons [7].…”

Exosomal circRNAs: new players in the field of cholangiocarcinoma

“…Nevertheless, points of consensus are starting to emerge in the literature. For instance, one of the latest studies in the field strengthened the established process governing circRNA biogenesis (7). Thus, it was confirmed that circRNAs are natural byproducts of spliceosomemediated splicing.…”

Pre-mRNA Splicing in the Nuclear Landscape

“…The CBC helps to promote E complex formation by interacting directly with U1 to recruit the snRNP and stabilize it at the 5′ss to promote spliceosome assembly (Fig. 4A,B; Lewis et al 1996;Gornemann et al 2005;Larson and Hoskins 2017;Li et al 2019). Furthermore, the CBC has been proposed to play a role in the stable recruitment of the tri-snRNP (Pabis et al 2013).…”

“… Cap-binding complex aids in U1 recruitment. ( A ) The structure of the spliceosome E complex showed physical interactions with CBC and the U1 snRNP component Snp1 (U1–70K in humans; pdb 6N7P) ( Li et al 2019 ). ( B ) Capping of the nascent RNA recruits CBC, which in turn can nucleate spliceosome assembly and splicing.…”