Fig. 3. Detailed views of the three stable conformations
observed for the 288–292 loop. The loop is located at the
base of the middle finger motif (orange β-strands) and
forms several hydrogen bonds (cyan) with the base of the
ring finger (yellow). (A) The in/up conformation involves
four hydrogen bonds to the ring finger and an Asp177–
Lys61 salt bridge between the ring and index finger. (B)
The in/down conformation is characterized by a Gly289-
dependent rotation of Ser288 whose side chain now
interacts with the NTP-binding Asp238 residue, resulting in
the loss of three hydrogen bonds to the ring finger and
disruption of the Asp177–Lys61 salt bridge. (C) The out/
down conformation has Cys290 flipped out of its hydrophobic
pocket, and as a result, the loop protrudes out into
the template RNA channel.