FIGURE 1.

(A) Parallels between the splicing mechanisms of the spliceosome and group II introns. Group II intron and mRNA precursor splicing using branch and hydrolytic pathways are depicted, together with the diastereomeric preference (Rp or Sp) for each step of the splicing reaction. (B) The full “six-wheel” domain structure of group II introns. (C) A schematic of the tripartite model system. (D–F) Details of the secondary structures of the components of the new tripartite model system developed to probe structure and function. (D) D123 and D5 secondary structures. Major tertiary interactions between domains 1, 2, 3, and 5 are depicted. (E) Schematic of 3D structure of D5-PL with same color coding as the D5 secondary structure. Major elements of the tetraloop, bulge, and catalytic triad are shown. (F) E1E2 substrate sequence. The substrate cleavage site is indicated by “X”.