A26. A26 forms three hydrogen bonds with stem 2. Two hydrogen bonds from the N3 and hydroxyl group of A23 to the C16 2 0 OH together form a single ribose-zipper motif. The third hydrogen bond is between the N1 and amino group of G4. A pyrimidine seems too small to fulfil these interactions and indeed replacing A26 by C or U showed an approximately 3-fold lower frameshifting activity (Table 1, SF217 and SF216 ). G would be capable of forming the ribose zipper motif though a considerable clash between its N1 and the G4 amino group can be expected. The relatively high frameshifting activity, 70% of the A26G pseudoknot, suggests that, at this position in L2, stacking properties assisted by the ribose interaction may be more important than formation of the zipper motif itself.