Time-resolved, single-molecule, correlated chemical probing of RNA

Methods: for capturing the folding dynamics of functionally important RNAs, especially large RNAs, have relied primarily on global measurements of structure or on per-nucleotide chemical probing. These approaches infer, but do not directly measure, through-space tertiary interactions. Here we introduce trimethyloxonium (TMO) as a chemical probe for RNA. TMO enables time-resolved, single-molecule, through-space structure probing of RNA folding using a correlated chemical probing framework. TMO methylates RNA about 90 times faster than the widely used dimethyl sulfate probe, allowing structure interrogation on the second time scale. We used TMO to monitor folding of the RNase P RNA – a functional RNA with extensive long-range and noncanonical interactions – by direct measurement of through-space tertiary interactions in a time-resolved way. Time-dependent correlation changes directly revealed the central role of a long-range tertiary loop-loop interaction that guides native RNA folding. Single-molecule, time-resolved RNA structure probing with TMO is poised to reveal a wide range of dynamic RNA folding processes and principles of RNA folding.