Both splice sites are thus defined at this early stage of the reaction. In this complex, the U1 snRNP binds the 5′SS via base pairing between U1 snRNA and the 5′SS, and the 3′SS and the PPT are associated with a heterodimer of U2AF1 (U2AF35) and U2AF2 (U2AF65) 7, 8. The first step in spliceosome assembly is the formation of the commitment complex. The PPT and the branch site are located upstream of intronic 3′ ends 6. The splicing reaction is governed by four main regulatory consensus sequences: the 5′ and the 3′ splice sites (5′SS and 3′SS, respectively), which are located at exon–intron boundaries, the polypyrimidine tract (PPT), and the branch site sequence.
Splicing is carried out within the spliceosome, a multi-component complex composed of five nuclear ribonucleoprotein (snRNP) complexes-U1, U2, U4, U5, and U6-and many additional proteins 4, 5. The splicing machinery recognizes either exons or introns as the spliced unit, through mechanisms called exon definition and intron definition, respectively 3. Splicing is the mRNA maturation reaction where introns are removed from pre-mRNA and exons are ligated together 1, 2.
