Abstract
The U2 and U6 snRNAs contribute to the catalysis of intron removal while U5 snRNA loop 1 holds the exons for ligation during pre-mRNA splicing. It is unclear how different exons are positioned precisely with U5 loop 1. Here, we investigate the role of U2 and U6 in positioning the exons with U5 loop 1. Reconstitution in vitro of spliceosomes with mutations in U2 allows U5-pre-mRNA interactions before the first step of splicing. However, insertion in U2 helix Ia disrupts U5-exon interactions with the intron lariat-3' exon splicing intermediate. Conversely, U6 helix Ia insertions prevent U5-pre-mRNA interactions before the first step of splicing. In vivo, synthetic lethal interactions have been identified between U2 insertion and U5 loop 1 insertion mutants. Additionally, analysis of U2 insertion mutants in vivo reveals that they influence the efficiency, but not the accuracy of splicing. Our data suggest that U2 aligns the exons with U5 loop 1 for ligation during the second step of pre-mRNA splicing.