Mechanical Unfolding of the Beet Western Yellow Virus -1 Frameshift Signal

Mechanical unfolding of -1 frameshift signals such as RNA pseudoknots have aimed to test the hypothesis that the stability of the pseudoknot is directly correlated to the frameshifting efficiency. Here we report unfolding of the Beet Western Yellow Virus (BWYV) pseudoknot by optical tweezers experiments complemented by computer simulations using steered molecular dynamics (SMD). Seven pseudoknot scenarios were studied: the wild-type pseudoknot in the presence and absence of Mg2+, the wild-type pseudoknot at high pH (deprotonated C8), and C8U, C8A, A24G, G19U, and G19UC mutant constructs. The mutants were selected to probe three key structural features of the BWYV pseudoknot, a triple-stranded helix at the base of stem 1, the stem junction region of stem 1 and stem 2, and a unique quadruple base-pair interaction involving a protonated cytosine in position 8 (C8). These regions are thought to control ribosomal frameshifting by different strategies such as thermodynamic stability, kinetic influences, and dynamics involving contacts with the ribosome. In addition, the mutants have been shown to either abolish frameshifting ability of the pseudoknot (C8 mutant cases and A24G), or actually increase the frameshifting efficiency (as seen with G19U and G19UC). We find three major conclusions from the stretching of the pseudoknot constructs with optical tweezers. First, stretching in the absence of Mg2+ results in no observed unfolding transitions. We interpret this to mean that magnesium is indispensible for the stable folding of the pseudoknot. Second, we found that frameshifting efficiency is not correlated with the force required to unfold the pseudoknots. However, we observe the unfolding of stem 1 in all of the pseudoknots stretched, where stem 2 unfolding is below our noise level. For this reason, we cannot rule out the possibility that an estimate of the thermodynamic stability of the entire pseudoknot would correlate with frameshifting efficiency. And third, we found that each pseudoknot mutant that resulted in reduced frameshifting efficiency also exhibited more off-equilibrium unfolding transitions that the wild-type pseudoknot under comparable loading rates. We conclude from these studies that the resistance of a pseudoknot to unfolding is controlled by both thermodynamic and kinetic parameters. We then suggest new technologies that would allow for greater resolution in order to correlate pseudoknot unfolding behavior with -1 programmed ribosomal frameshifting events