The TPP riboswitch. (a) Structure of the riboswitch RNA bound to its ligand, thiamine pyrophosphate (TPP). (b) Proposed mechanism of TPP riboswitch function. When bound to TPP, the riboswitch assumes a conformation that sequesters the Shine-Dalgarno sequence, preventing translation of the mRNA. Mg²⁺ ions (not shown) also play a role, allowing closer packing of the RNA helices and TPP. In the absence of TPP, an alternative secondary structure forms (yellow regions are complementary to and base-pair with each other), and translation can be initiated.