Lysine
Lysine riboswitch: PDB ID
The lysine riboswitch is one of the largest known riboswitches. Its strucure is essentially a "bundle" of three sets of coaxially stacked helices with lysine embedded within a five-way junction.
The unbound lysine riboswitch: PDB ID
This structure represents the first published structure of the unliganded form of a riboswitch. Notably, it is almost identical in structure to the bound form, demonstrating that the free state can at least transiently sample "bound-like" states. This was the first evidence that riboswitches utilize a conformational selection binding mechanism, which has now been shown for a variety of other riboswitch families.