They are always found in the apparent 5' untranslated regions of metK genes, which encode the enzyme (Methionine adenosyltransferase) that synthesizes SAM.

However, the ability to reject SAH as a ligand might not be important under physiological conditions, because the cellular concentration of SAM is higher.

[2] A region of the conserved structure of SAM–SAH riboswitches includes a predicted Shine-Dalgarno sequence (ribosome-binding site) of the downstream metK genes.

These nucleotides are required for optimal binding to the ligand and might form a pseudoknot with the terminal loop within the main stem-loop structure.

Occlusion of the Shine-Dalgarno sequence might be the mechanism by which SAM–SAH riboswitches regulate expression of the downstream genes.