Figure 6

Models of the 3′-spliced junction. The junction (red ribbon) is constrained into the active site by two sets of interactions: the γ-γ' interaction that joins the last intron nucleotide (U412) with A287 in the J2/3 junction (intron nucleotides are depicted in green), and the EBS3-IBS3 interaction that joins the first nucleotide of the 3′-exon (U+1); with A223 in D1. The scissile phosphate (P) presents its pro-S oxygen to the catalytic metals (M1-M2, yellow spheres). The backbone of the junction may be kinked (angle of 50° between the two phosphate groups that flank the scissile phosphate, left panel, [29]) or elongated (angle of 134°, right panel).