It was patented in the US in the 1950s by Colgate, though Biolipid transesterification may have been discovered much earlier.
Complementary binding between U2 snRNA (in an area lying towards the 5' end but 3' to hairpin I) and the branchpoint sequence (BPS) of the intron results in the bulging out of an unpaired adenosine, on the BPS, which initiates a nucleophilic attack at the intronic 5' splice site, thus starting the first of two transesterification reactions that mediate splicing.