The Dynamic Nature of RNA as Key to Understanding Riboswitch Mechanisms

Riboswitches are gene regulation elements within RNA that recognize specific metabolites. They predominantly occur in the untranslated leader regions of bacterial messenger RNA (mRNA). Upon metabolite binding to the aptamer domain, a structural change in the adjoining downstream expression platform signals "on" or "off" for gene expression. Researchers have achieved much progress in characterizing ligand-bound riboswitch states at the molecular level; an impressive number of high-resolution structures of aptamer-ligand complexes is now available. These structures have significantly contributed toward our understanding of how riboswitches interact with their natural ligands and with structurally related analogues. In contrast, relatively little is known about the nature of the unbound (apo) form of riboswitches. Moreover, the details of how changes in the aptamer domain are transduced into conformational changes in the decision-making expression platform remain murky. In this Account, we report on recent efforts aimed at the characterization of free states, ligand recognition, and ligand-induced folding in riboswitches. Riboswitch action is best approached as a cotranscriptional process, which implies sequential folding and release of the aptamer prior to the signaling of the expression platform. Thus, a complex interplay of several factors has to be taken into account, such as speed of transcription, transcriptional pausing, kinetics and thermodynamics of RNA structure formation, and kinetics and thermodynamics of ligand binding. The response mechanism appears to be best described as a process in which ligand recognition critically dictates the folding pathway of the nascent mRNA during its expression; the resulting structures determine the interactions with the transcriptional or translational apparatus. We discuss experimental methods that offer insight into the dynamics of the free riboswitch state. These include probing experiments, such as in-line and selective 2'-hydroxyl acylation analyzed by primer extension (SHAPE) techniques, small-angle X-ray scattering (SAXS) analysis, NMR spectroscopy, and fluorescence spectroscopy, including single-molecule fluorescence resonance energy transfer (smFRET) imaging. One of our research contributions is an approach, termed 2ApFold, that incorporates noninvasive 2-aminopurine modifications in riboswitches. The fluorescence response of these moieties is used to delineate the order of secondary-tertiary structure formation and rearrangements taking place during ligand-induced folding. This information can be used to explore the kinetics of ligand recognition and to analyze the degree of structure preorganization of the free riboswitch state. Furthermore, we discuss a recent smFRET study on the SAM-II riboswitch; this report underscores the importance of choosing strategic labeling patterns that leave maximal conformational freedom to the regulatory interaction. Finally, we comment on how riboswitch ligand recognition appeals to the concepts of conformational selection and induced fit, and on the question of whether riboswitches act under thermodynamic or kinetic control. This Account highlights the fact that a thorough understanding of RNA dynamics in vitro is required to shed light on cellular riboswitch mechanisms. Elucidating these mechanisms will contribute not only to ongoing efforts to target riboswitches with antibiotics but also to attempts to engineer artificial cell regulation systems.