The removal of introns from precursor messenger RNAs occurs in a large complex, the spliceosome, that contains many proteins and five small nuclear RNAs (snRNAs). The snRNAs interact with the intron-containing substrate RNA and with each other to form a dynamic network of RNA interactions that define the intron and promote splicing. There is evidence that protein splicing factors play important roles in regulating RNA interactions in the spliceosome. PRPS is a highly conserved protein that is associated in particles with the U5 snRNA and directly binds the substrate RNA in spliceosomes. UV crosslinking has been used to map the binding sites, and shows extensive interaction between PRPS protein and the 5′ exon prior to the first step of splicing and with the 3′ splice site region subsequently. It is proposed that PRPS protein may stabilize fragile interactions between the U5 snRNA and exon sequences at the splice sites, to anchor and align them in the catalytic centre of the spliceosome.