FIGURE 1.

(A) Cartoon of regulation by riboswitches. When ligand binds to the aptamer domain (cyan), this incorporates a “switching sequence” at the 3′ side of the P1 helix (red) into this domain, forcing the downstream expression platform (orange) to form a terminator element. In the absence of ligand, the “switching sequence” is available to be used to form an antiterminator that in turn allows for synthesis of the full-length mRNA. Secondary (B) and tertiary (C) structure of the aptamer domain of the B. subtilis xpt-pbuX guanine riboswitch aptamer domain bound to hypoxanthine (red) (PDB ID 1U8D).