Spliceosomal Active C Complex Spliceosomal active C complex with lariat containing, 5'-end cleaved pre-mRNP:CBC complex Lariat Formation and 5'-Splice Site Cleavage Authored: Krainer, AR, 2003-06-05 08:30:23 In the first catalytic step of mRNA splicing, the 2' OH group of the bulged A at the branch site performs a nucleophilic attack on the 5' splice site phosphodiester bond, resulting in cleavage of the bond between the 5' exon and the 5' end of the intron, and formation of a new bond between the 5' end of the intron and the branch site A. This results in a lariat-shaped intermediate, with the intron still attached to the 3' exon. The branch site A has a 2'-5' phosphodiester bond with the G at the beginning of the intron, in addition to the usual 5'-3' and 3'-5'phosphodiester bonds. Pubmed6088074 Pubmed6206566 Reactome Database ID Release 4372143 Reactome, http://www.reactome.org ReactomeREACT_1935