The U1, U2 and U5 snRNAs crosslink to the 5' exon during yeast pre-mRNA splicing

Activation of pre-messenger RNA (pre-mRNA) splicing requires 5' splice site recognition by U1 small nuclear RNA (snRNA), which is replaced by U5 and U6 snRNA. Here we use crosslinking to investigate snRNA interactions with the 5' exon adjacent to the 5' splice site, prior to the first step of splicing. U1 snRNA was found to interact with four different 5' exon positions using one specific sequence adjacent to U1 snRNA helix 1. This novel interaction of U1 we propose occurs before U1-5' splice site base pairing. In contrast, U5 snRNA interactions with the 5' exon of the pre-mRNA progressively shift towards the 5' end of U5 loop 1 as the crosslinking group is placed further from the 5' splice site, with only interactions closest to the 5' splice site persisting to the 5' exon intermediate and the second step of splicing. A novel yeast U2 snRNA interaction with the 5' exon was also identified, which is ATP dependent and requires U2-branchpoint interaction. This study provides insight into the nature and timing of snRNA interactions required for 5' splice site recognition prior to the first step of pre-mRNA splicing.