FIGURE 10-11 Model of spliceosome-mediated splicing of pre-mRNA. Step 1: After U1 base-pairs with the consensus 5′ splice site, SF1 (splicing factor 1) binds the branch-point A; U2AF (U2 snRNP associated factor) associates with the polypyrimidine tract and 3′ splice site; and the U2 snRNP associates with the branch-point A via base-pairing interactions shown in Figure 10-9, displacing SF1. Step 2: A trimeric snRNP complex of U4, U5, and U6 joins the initial complex to form the spliceosome. Step 3: Rearrangements of base-pairing interactions between snRNAs convert the spliceosome into a catalytically active conformation and destabilize the U1 and U4 snRNPs, which are released. Step 4: The catalytic core, thought to be formed by U6 and U2, then catalyzes the first transesterification reaction, forming the intermediate containing a 2′,5′-phosphodiester bond, as shown in Figure 10-8. Step 5: Following further rearrangements between the snRNPs, the second transesterification reaction joins the two exons by a standard 3′,5′-phosphodiester bond and releases the intron as a lariat structure as well as the remaining snRNPs. Step 6: The excised lariat intron is converted into a linear RNA by a debranching enzyme. See T. Villa et al., 2002, Cell 109:149.