Model of electrostatic stabilization by a cationic nucleobase. The exogenous nucleobase stacks between A7 and A9 in the pocket left by the G8 deletion, restoring active site architecture. The amidine group of the rescuing nucleobase forms hydrogen bonds with 2' hydroxyl and phosphoryl oxygens. These interactions serve to stabilize negative charge that develops in the transition state and to position reactive groups in the orientation appropriate for an SN2 in-line nucleophilic attack.