Mechanisms of action of a riboswitch

In the absence of effector (upper left) the ligand-binding site (L) is unoccupied, and a transcriptional antiterminator (AT) can form.

Alternatively, if the riboswitch functions at the level of translation (lower left), a region containing the Shine-Dalgarno sequence (SD) is not paired with its complement (anti-SD, or ASD), and translation can occur.

In the presence of an effector, the mRNA tertiary structure can change to pair the antiterminator sequence with its complement (AAT), allowing a terminator loop (T) to form, leading to transcriptional termination (upper right).

Alternatively (lower right), binding the ligand permits the Shine-Dalgarno sequence to pair with its complement (ASD) so that translation cannot initiate.