SMK box riboswitch

The SMK box riboswitch is an RNA element that regulates gene expression in bacteria. The SMK box riboswitch is found in the 5' UTR of the MetK gene in lactic acid bacteria. The structure of this element changes upon binding to S-adenosyl methionine (SAM) to a conformation that blocks the shine-dalgarno sequence and blocks translation of the gene.

There are two other known SAM-binding riboswitches: SAM-I, SAM-II, but these appear to share no similarity in sequence or structure.

Structure
This riboswitch is composed of a single stem-loop. In some species there are large insertions of up to 210 nucleotides within this structure.