In a series of control experiments, 8 nt oligos complementary to the 5 0 (Loop 1) and 3 0 Stem 2 forming regions of the pseudo-pseudoknot were hybridized to the SL mRNA and À1 PRF assays were performed. Neither of these were able to stimulate À1 PRF, even at concentrations in 100-fold molar excess to the mRNA template (data not shown). Though supportive of our central hypothesis, it is also possible that these results were due to the thermodynamic instability of the RNA:DNA duplexes through the course of the experimental protocol.