The SMKbox riboswitch (also known as SAM-III) is an RNA element that regulates gene expression in bacteria.

[2][3] The SMK box riboswitch is found in the 5' UTR of the MetK gene in lactic acid bacteria.

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.