Functional characterization of dynamic nascent RNA folding ensembles in real time

RNA structure starts forming cotranscriptionally as the nascent RNA emerges from the RNA polymerase and is dynamically modulated by cellular factors. How individual RNA conformations, out of an ensemble of RNA molecules, relate to function is not well understood. Here, developing multicolor single-molecule fluorescence microscopy experiments, we track in real time nascent RNA structure formation, functionally characterizing up to eight different types of RNA molecules. We find that ribosomal proteins, RNA modification enzymes or antisense oligonucleotides specifically modulate a subset of the RNA folding classes. For example, we provide direct evidence that increased local RNA accessibility at specific sites correlates with the chaperoning activity of ribosomal proteins during ribosome assembly. These experiments provide a general framework to study how dynamic RNA folding, and misfolding, relates to function.