Figure 5. Cytosines and uracils stack to form a helical conformation in the Pan2 active site.
a, Circular dichroism spectra of A6 (green), AAUUAA (purple), and AACCAA (blue) RNAs. These spectra are representative of identical experiments performed 2 times. b-c, Crystal structure of Pan2UCH-Exo E912A (light blue) bound to AACCAA (dark blue, b) or AAUUAA (purple, c) RNA. The electron density of RNA bound in the active site is visualized with feature-enhanced maps contoured to 1.8σ. d, Superposition of A7 (green), AAUUAA (purple), and AACCAA (blue) RNAs. The bases in C- and U-containing RNAs are relatively well-stacked compared to G-containing RNAs (Fig. 4b) and are thus permissive to form the single-stranded RNA helix required for Pan2 activity.