first spliceosomal transesterification activity
Catalysis of the first transesterification reaction of 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, while it is still joined to the 3' exon.
GO:0140098
catalytic activity, acting on RNA
is_a
lariat formation, 5'-splice site cleavage
related
SPBC1711.17