RNA molecules must fold into specific three-dimensional shapes to perform their structural and catalytic functions. Unlike proteins, RNAs secondary structural features are usually stable enough to form by themselves in solution. The reason is that in RNA, the stabilization energy gained from the formation of secondary structure is substantially larger than the energies involved in tertiary interactions. As a result, the formation of tertiary interactions is expected to alter only slightly the pre-existing secondary structural contacts. Moreover, secondary structure prediction is robust and can be made without taking into consideration tertiary folding. However, bulk studies of the energetics and kinetics of their secondary and tertiary folding are often frustrated by the presence of multiple species and multiple folding pathways in solution. These problems are circumvented in single-molecule studies in which the folding/unfolding trajectories of the individual molecules can be followed. The T. thermophila group I intron ribozyme is organized into several domains whose mechanical unfolding can be investigated independently, and whose tertiary contacts are stabilized by numerous Mg++ ions.

We have begun characterization of the ribozyme by analysis of the P5abc domain because: