part of RNA splicing, via transesterification reactions second spliceosomal transesterification activity Catalysis of the second transesterification reaction of spliceosomal mRNA splicing. Ligation of the two exons occurs via a transesterification reaction where the free 3'-hydroxyl group of the 5' exon is the nucleophile attacking the 3' splice site. Non-expressed sequences are now detached from the exons. In cis splicing, the intron is in a lariat structure. 3'-splice site cleavage, exon ligation lariat formation, 5'-splice site cleavage GO:0000386 Reactome:R-HSA-9770236 molecular_function catalytic activity, acting on RNA