FIG 13.
Poliovirus 3Dpol/RNA elongation complex. (a) The lengthening RNA double helix protrudes from the right side of the polymerase. Orientation and coloring are as shown Fig. 12b. (b) Zoomed image of the active site in its closed conformation. The incoming rNTP (C) and its paired template base (not shown) swing down, displacing D-238, which frees up N-297 and positions S-288 so that both residues hydrogen bond (dashed lines) to the 2′ OH group of the incoming rNTP. In addition, a three-strand beta sheet (upper right) is stabilized, positioning D-233 to coordinate both catalytic magnesium ions (spheres). One of the magnesium ions stabilizes the leaving pyrophosphate group (PP).