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Cooperative and directional folding of the preQ1 riboswitch aptamer domain.

Riboswitches are cis-acting RNA fragments that regulate gene expression by sensing cellular levels of the associated small metabolites. In bacteria, the class I preQ(1) riboswitch allows the fine-tuning of queuosine biosynthesis in response to the intracellular concentration of the queuosine anabolic intermediate preQ(1). When binding preQ(1), the aptamer domain undergoes a significant degree of secondary and tertiary structural rearrangement and folds into an H-type pseudoknot. Conformational "switching" of the riboswitch aptamer domain upon recognizing its cognate metabolite plays a key role in the regulatory mechanism of the preQ(1) riboswitch. We investigate the folding mechanism of the preQ(1) riboswitch aptamer domain using all-atom Go̅-model simulations. The folding pathway of such a single domain is found to be cooperative and sequentially coordinated, as the folding proceeds in the 5' → 3' direction. This kinetically efficient folding mechanism suggests a fast ligand-binding response in competition with RNA elongation.