Lysine riboswitch

The Lysine riboswitch is a metabolite binding RNA element found within certain messenger RNAs that serve as a precision sensor for the amino acid lysine. Allosteric rearrangement of mRNA structure is mediated by ligand binding, and this results in modulation of gene expression. Lysine riboswitch are most abundant in Firmicutes and Gammaproteobacteria where they are found upstream of a number of genes involved in lysine biosynthesis, transport and catabolism. The lysine riboswitch has also been identified independently and called the L box.
Structure.
The structure of the lysine riboswitch has recently been determined. The lysine amino acid is bound in the pocket formed by the 5-way junction. The structure is composed of a three helical bundle and a two helical bundle joined by the 5-way junction. Helices 1 and 2 are stacked in a colinear fashion as are helices 4 and 5.