The SMKbox riboswitch (also known as SAM-III) is a 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 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.