Figure 5. Step 1 factors and branch site positioning.
a, interaction between the RNA catalytic core and Prp8. b, positioning of the branch helix by step 1 factors. c, corresponding view in S.pombe post splicing ILS complex26, showing dramatic repositioning of the branch helix and its further stabilisation by debranching co-factor Cwf19. d, a close-up view of step 1 factors interacting with the branch helix.