The results presented in this study provide strong support for the torsional restraint model of programmed À1 frameshifting. Specifically, we demonstrated that RNA:DNA hybrids that mimic mRNA pseudoknots can significantly stimulate frameshifting. As predicted by the model, changing the rotational freedom of the structure by altering the lengths in the J1-and R1-oligos between the 5 0 and 3 0 mRNA hybridizing regions resulted in changes in their abilities to stimulate À1 frameshifting. The demonstration that these 'pseudopseudoknot' structures cause elongating ribosomes to specifically pause with their A-and P-sites positioned at the slippery site provides independent evidence in support of the model. In the case of the J-oligo series, frameshifting was best stimulated by J2, suggesting the structure created and the rotational freedom allowed by it was optimal for À1 PRF. The experimental design is such that we assume a similar rate of unfolding for each oligo as the predicted maximum base pairing is the same for them all. However, we do note that the type of nucleotides separating the two, separately paired regions of the oligos, and their presentation, may play a role in À1 PRF efficiency. The recent NMR structural solution of the SRV-1 pseudoknot revealed a highly structured Loop 2-Stem 1 interface including base triples involving an A residue at the 3 0 end of Loop 2 (26) . The additional base in the J2oligonucleotide is also an A. Mutagenesis experiments in this region by other groups showed, for example, that replacing the 3 0 base in Loop 2 of IBV with an A residue promoted a significant increase in frameshifting efficiency (27) , and mutation or removal of the A residue at the 3 0 base in Loop 2 of the BWYV pseudoknot reduced frameshifting levels (17) . This part of the pseudoknot has been proposed to be important in a frameshifting model where differential transition state energy barriers (due to small differences in local structure, stability or dynamics) are the primary determinant of frameshifting efficiency (3). Indeed, a Loop 2-Stem 1 triplex interaction seen in smaller frameshifting pseudoknots from luteoviruses has been shown to be critical for À1 PRF, and that similar pseudoknots lacking the triplex are less efficient at frameshifting [(28) and references therein]. This extra structural feature would limit the rate of unfolding and provide extra anchoring of Stem 2 as the ribosome attempts to unwind Stem 1, i.e. it too would help to provide additional torsional restraint. It is also possible that although the J3-and J4oligonucleotides also help to form a pseudo-pseudoknot, the additional bases may interfere with the stabilization of Stem 1.