OBSOLETE. Catalysis of the first transesterification reaction of U2-type spliceosomal mRNA splicing. The intron branch site adenosine is the nucleophile attacking the 5' splice site, resulting in cleavage at this position. In cis splicing, this is the step that forms a lariat structure of the intron RNA, which is still joined to the 3' exon. The catalytic site is thought to be formed by U6 and/or U2 snRNA, and/or associated proteins.