Exploring RNA folding one molecule at a time

RNA molecules fold into stable native structures to perform their biological function. RNA folding can be influenced by ions, co-factors, and proteins through numerous mechanisms. Understanding these mechanisms at the molecular level is important for elucidating the structure–function relationship in biologically important RNAs. Recent developments in single molecule spectroscopy have provided new approaches to investigate RNA folding and have allowed identification of kinetic intermediates that would otherwise remain hidden in ensemble-averaged experiments. Here we summarize some of these developments, which provide new insight into the effect of Mg 2+ ions in RNA folding landscapes, the role of cooperativity in RNA tertiary folding, the stepwise folding of RNA during transcription, and the hierarchical assembly of RNA–protein complexes.