The bound lysine riboswitch (green) superimposed upon the unbound riboswitch (orange). Note that the binding pockets conformation is nearly identical in both the liganded and unliganded forms.
Lysine riboswitch: PDB ID3D0U
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 3D0X
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.