Figure 3. A riboswitch timeline.

During the aptamer time regime (black), the sensor domain is synthesized and acquires secondary structure rapidly followed by slower formation of Mg²⁺-dependent tertiary structure. Upon formation of a competent aptamer domain the opportunity for ligand binding becomes possible. During the expression time regime (blue) the expression platform is synthesized, but the aptamer domain remains binding competent until the riboswitch has committed to forming the antiterminator element (“ligand association time”). During this time there are often two programmed transcriptional pauses (boxes 1 and 2) that stall the polymerase, and thereby influence the amount of time the riboswitch has for sensing its environment before committing to formation of one of the two mutually exclusive secondary structures in the expression platform.