Figure 6. Schematic showing proposed regulation mechanisms for add and pbuE adenine riboswitches.

Top, regulation mechanism of the add adenine riboswitch. The OFF state is represented with the Shine-Dalgarno (GAA) and AUG start codon sequences base paired in the sequestrator helix. Upon adenine (Ade) binding, the ON state is formed which increases the accessibility of both GAA and AUG sequences. The structural reversibility and the lack of requirement of a transcription-translation coupling for the regulatory activity of the riboswitch are consistent with a thermodynamic regulation regime. Bottom, regulation mechanism of the pbuE adenine riboswitch. In this regulation mechanism, a low intracellular adenine concentration leads to the formation of the OFF state. However, an elevated adenine concentration may co-transcriptionally bind to the riboswitch aptamer on a paused transcription complex, thereby stabilizing the aptamer, which will then ultimately lead to the formation of the ON state and the expression of pbuE. Because adenine binding occurs co-transcriptionally and is largely dependent on the rate of transcription, the regulation mechanism is consistent with a kinetic regime. Aptamers having thick lines represent stabilized complexes in presence of adenine.