The S<sub>MK</sub>box riboswitch (also known as SAM-III) is an RNA element that regulates gene expression in bacteria. The S<sub>MK</sub> 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 other known SAM-binding riboswitches such as SAM-I and SAM-II, but these appear to share no similarity in sequence or structure to SAM-III.
The crystal structure of the riboswitch from E. faecalis was solved by X-ray crystallography. The structure showed that the most conserved nucleotides involved in SAM binding were organised around a junction between three helices. In some species there are large insertions of up to 210 nucleotides within this structure.