Fig. 1

Conserved nucleotides (A) and schematic representation of spliceosomal intron excision (B). Branchpoint, polypyrimidine tract, as well as both 5’ and 3’ splice sites are indicated. All of them play a key role in the process of removing spliceosomal introns, which consist of two sequential transesterification reactions. (1) The 2' hydroxyl (2’OH) group of a branch site performs a nucleophilic attack on the 5' splice site, leading to the formation of the lariat intermediate. (2) The released 3'OH group from the exon conducts a nucleophilic attack on the 3' splice site, leading to exons joining and excision of the intron lariat