Characterization of the mechanical unfolding of RNA pseudoknots |
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Authors: | Green Lisa Kim Chul-Hyun Bustamante Carlos Tinoco Ignacio |
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Institution: | 1 Department of Chemistry, University of California, Berkeley, Berkeley, CA 94720, USA 2 Department of Chemistry and Biochemistry, California State University, East Bay, Hayward, CA 94542, USA 3 Department of Physics and Molecular and Cell Biology, University of California, Berkeley, Berkeley, CA 94720, USA 4 Howard Hughes Medical Institute, University of California, Berkeley, Berkeley, CA 94720, USA |
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Abstract: | The pseudoknot is an important RNA structural element that provides an excellent model system for studying the contributions of tertiary interactions to RNA stability and to folding kinetics. RNA pseudoknots are also of interest because of their key role in the control of ribosomal frameshifting by viral RNAs. Their mechanical properties are directly relevant to their unfolding by ribosomes during translation. We have used optical tweezers to study the kinetics and thermodynamics of mechanical unfolding and refolding of single RNA molecules. Here we describe the unfolding of the frameshifting pseudoknot from infectious bronchitis virus (IBV), three constituent hairpins, and three mutants of the IBV pseudoknot. All four pseudoknots cause −1 programmed ribosomal frameshifting. We have measured the free energies and rates of mechanical unfolding and refolding of the four frameshifting pseudoknots. Our results show that the IBV pseudoknot requires a higher force than its corresponding hairpins to unfold. Furthermore, its rate of unfolding changes little with increasing force, in contrast with the rate of hairpin unfolding. The presence of Mg2+ significantly increases the kinetic barriers to unfolding the IBV pseudoknot, but has only a minor effect on the hairpin unfolding. The greater mechanical stability of pseudoknots compared to hairpins, and their kinetic insensitivity to force supports the hypothesis that −1 frameshifting depends on the difficulty of unfolding the mRNA. |
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Keywords: | RNA pseudoknots single molecules frameshifting laser tweezers force unfolding |
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