Fig. S5.

Side view tracing the path of the flap through the narrow space under arch in f₅I (protein surface is shown, but the bottom part of α4 is omitted for clarity). The f2 base is flipped and contacts the active-site residues. The f3 base is also flipped and is in contact with the hydrophobic pocket formed between α2, α6, and β3. The C-terminal region forms a steric block at the arch exit. The f4 base does not form extensive contacts with the protein and is pointing to the normal orientation.