FIGURE 1.

Mechanism for RNA cleavage by internal phosphoester transfer involving the 2′-hydroxyl group. The RNA linkage (1) passes through a pentacoordinate species (2) that degrades into fragments that carry either a 2′,3′-cyclic phosphate terminus (3) or a 5′-hydroxyl terminus (4). The four catalytic strategies that can influence the reaction are identified as follows: α, in-line nucleophilic attack (blue); β, neutralization of negative charge on a nonbridging phosphate oxygen (purple); γ, deprotonation of the 2′-hydroxyl group (red); and δ, neutralization of negative charge on the 5′-oxygen atom (green).