Riboswitch. (A) The 5′-end of an mRNA that encodes proteins engaged in the production of flavin mononucleotide (FMN) folds to form a structure that is stabilized by binding FMN. This structure includes a terminator that leads to premature termination of the mRNA. At lower concentrations of FMN, an alternative structure that lacks the terminator is formed, leading to the production of full-length mRNA. (B) The three-dimensional structure of a related FMN-binding riboswitch bound to FMN. The blue and yellow stretches correspond to regions highlighted in the same colors in part A. Notice how the yellow strand contacts the bound FMN, stabilizing the structure.

[Drawn from 3F2Q.pdb].