The interactions between L2 and S1 in the SRV-1 pseudoknot as previously determined by NMR (26) turn out to be important for frameshifting. Although single alterations of the base triples found in the NMR study had at most a 3-fold deleterious effect on frameshifting, combining several of these changes into one mutant, L2-mut, led to an 8-fold decrease in vitro and a 6-fold decrease in vivo. This is quite remarkable as the mutated sequence should still be able to fold as a pseudoknot, but apparently critical stem-loop interactions are lacking. As found for other frameshifter pseudoknots, the composition and length of L2 can greatly influence frameshifting. Most frameshifter pseudoknots have adenine-rich L2 loops, presumably because of the need for stable triple helix formation, which is best performed by adenosine's N1 and C6-amino groups (2) .