Figure 5. Comparison between thermal and mechanical (un)folding pathways and scheme of ribosomal −1 frameshifting.

(a) Proposed thermal unfolding and folding pathways (see Fig. 2). (b) Proposed single-molecule mechanical unfolding and folding pathways. The stabilizing effect of coaxial stacking between stem 1 (red) and stem 2 (blue), and the base triples formed between stem 1 and loop 2 (green) on the component stems (stem 1 and stem 2) is revealed by analyzing the one-step mechanical unfolding forces. (c) Scheme of ribosomal frameshifting from 0 frame to −1 frame. The mRNA slippery sequence and single-stranded spacer within the ribosome are shown in yellow and gray, respectively. Stem 1-loop 2 base triple formation in the mRNA pseudoknot may enhance the local stability of stem 1 and result in enhanced frameshifting. The tRNAs and nascent peptide are shown in magenta and brown, respecitvely. The drawing is not to scale.