Linking folding dynamics and function of SAM/SAH riboswitches at the single molecule level
Riboswitches are found in the 5-UTR of many bacterial mRNAs. They function as cisacting regulatory elements that control downstream gene expression through ligand-induced conformational changes. Here, we used single-molecule FRET to map the conformational landscape of the SAM/SAH riboswitch and probe how co-transcriptional ligand-induced conformational changes of this translational switch alter ribosome accessibility. The folding of the riboswitch is highly heterogenous, indicating a complex and rugged conformational landscape that enables sampling of the ligand-bound conformation even in the absence of the ligand. Upon ligand binding, the landscape shifts towards the ligand-bound conformation. Mutations at key stabilizing structures alter the ligand-free folding behavior and decrease ligand responsiveness. We also explored translational regulation through folding kinetics by utilizing short oligonucleotides to probe the accessibility of the Shine-Dalgarno sequence within the riboswitch. Additionally, we employed a helicase-based vectorial folding assay to simulate co-transcriptional folding. We find that a competition between ligand binding and ribosome binding is fined tuned via the kinetics of folding. During transcription, the riboswitch takes minutes before reaching equilibrated conformations, and such slow equilibration decreases the effective ligand affinity. Overall, our data demonstrate the significance of conformational polymorphism in RNA function, emphasizing the utilization of complex folding landscapes for regulating ribosome accessibility through ligand induction. Furthermore, we provide direct evidence on how folding kinetics modulate this regulation process.