Little is known about how a non‐Watson–Crick pair affects the RNA folding dynamics. We studied the effects of a U⋅U‐to‐U⋅C pair mutation on the folding of a hairpin in human telomerase RNA. The ensemble thermal melting of the hairpins shows an on‐pathway intermediate with the disruption of the internal loop structure containing the U⋅U/U⋅C pairs. By using optical tweezers, we applied a stretching force on the terminal ends of the hairpins to probe directly the non‐nearest‐neighbour effects upon the mutations. The single U⋅U to U⋅C mutations are observed to 1) lower the mechanical unfolding force by approximately 1 picoNewton (pN) per mutation without affecting the unfolding reaction transition‐state position (thus suggesting that removing a single hydrogen bond affects the structural dynamics at least two base pairs away), 2) result in more frequent misfolding into a small hairpin at approximately 10 pN and 3) shift the folding reaction transition‐state position towards the native hairpin structure and slightly increase the mechanical folding kinetics (thus suggesting that untrapping from the misfolded state is not the rate‐limiting step).
The cover picture shows the experimental setup for mechanical (un)folding of RNA hairpins using optical tweezers (not to scale). The contribution of a single hydrogen bond to RNA folding was directly revealed by nanomanipulation using optical tweezers. Disruption of a single hydrogen bond by the replacement of a single RNA U⋅U pair with a U⋅C pair results in an increase in the rate of mechanical unfolding and more frequent misfolding. Details are given in the Full Paper by Gang Chen et al. on .
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