Fig. 2. Conformational changes of the RNA elements.
a Overall structure of the RNA elements in the pre-Bact complex. The color-coding scheme is preserved throughout this manuscript. In pre-Bact, U6 ISL and helices Ib and II of U2/U6 duplex are initially formed; but helix Ia is yet to form. The BS is ~60 Å away from 5′SS. The 5′-exon remains anchored to loop I of U5 snRNA. b Structure comparison of the RNA elements between the pre-Bact and Bact-I complexes. The U2/BS duplex and helix II remain unchanged. Helix Ia appears in Bact-I. The 5′SS moves toward the BS by ~10 Å, with notable movements for U5 and U6 snRNAs. All structure alignments reported in this manuscript are based on PRP8, unless stated otherwise. c Close-up views on the maturation of U6 ISL. In the pre-Bact to Bact-I transition, helix Ia is formed, and the key nucleotide U74 of U6 snRNA is bulged out. d Pairwise comparison of the RNA elements among the Bact-I, -II, -III, and -IV complexes. The distance between 5′SS and the BS remains unchanged at about 50 Å. In the transition of Bact-I to Bact-II, U2 snRNA undergoes a marked translocation and helix II is shifted slightly. In the transition of Bact-III to Bact-IV, helix II moves to the position where it is found in the C complex. e Structure comparison of the RNA elements between Bact-IV and post-Bact. U2 snRNA undergoes notable movements with the BS moving towards 5′SS by about 20 Å. f Structure comparison of the RNA elements between the post-Bact and C complexes. After the branching reaction, the BS is linked to 5′SS in the C complex, and U2 snRNA undergoes a pronounced translocation.