SRP199582 PRJNA544962 GSE131797 The impact of Prp18p on splicing fidelity and efficiency in budding yeast Fidelity of 3´-splice site (3´SS) selection by the spliceosome is critical for proper gene expression but is a daunting task considering the low complexity of the 3´SS consensus YAG. Here we show that loss of the splicing factor Prp18p in budding yeast activates a diverse array of alternative 3´SS at more than half of all introns. Some alternative sites highly diverge from the YAG consensus, demonstrating a critical role for Prp18p in promoting spliceosome fidelity. Usage of alternative 3´SS is determined by distance from the branchpoint, local RNA secondary structures, upstream poly(U) content, and adenosine enrichment in exons. The 3´SS fidelity function of Prp18p can be genetically uncoupled from its role in splicing efficiency and is promoted by interactions with Slu7p and Prp8p. Taken together, these results provide a comprehensive mechanism into how the spliceosome achieves specificity of 3´SS selection and how it prevents aberrant activation of non-canonical splice sites. Overall design: RNA sequencing of yeast strains with and without the second step splicing factor Prp18p in a nonsense-mediated decay mutant background GSE131797 pubmed 37956322