SAM-II riboswitch
The SAM-II riboswitch is an RNA element found predominantly in Alphaproteobacteria that binds S-adenosyl methionine. Its structure and sequence appear to be unrelated to the SAM [riboswitch (S box leader)|SAM riboswitch] found in Gram-positive bacteria. This SAM riboswitch is located upstream of the metA and metC genes in Agrobacterium tumefaciens, and other methionine and SAM biosynthesis genes in other alpha-proteobacteria. Like the other SAM riboswitch, it probably functions to turn off expression of these genes in response to elevated SAM levels. A significant variant of SAM-II riboswitches was found in Pelagibacter ubique and related marine bacteria and called SAM-V. Also, like many structured RNAs, SAM-II riboswitches can tolerate long loops between their stems.