SAH riboswitches are a kind of riboswitch

Riboswitch

In molecular biology, a riboswitch is a part of an mRNA molecule that can directly bind a small target molecule, and whose binding of the target affects the gene's activity. Thus, an mRNA that contains a riboswitch is directly involved in regulating its own activity, in response to the...

 that bind S-adenosylhomocysteine (SAH) . When the coenzyme S-adenosylmethionine (SAM) is used in a methylation

Methylation

In the chemical sciences, methylation denotes the addition of a methyl group to a substrate or the substitution of an atom or group by a methyl group. Methylation is a form of alkylation with, to be specific, a methyl group, rather than a larger carbon chain, replacing a hydrogen atom...

 reaction, SAH is produced. SAH riboswitches typically up-regulate genes involved in recycling SAH to create more SAM (or the metabolically related methionine

Methionine

Methionine is an α-amino acid with the chemical formula HO2CCHCH2CH2SCH3. This essential amino acid is classified as nonpolar. This amino-acid is coded by the codon AUG, also known as the initiation codon, since it indicates mRNA's coding region where translation into protein...

). This is particularly relevant to cells, because high levels of SAH can be toxic. Originally identified by bioinformatics, SAH riboswitches are apparent in many species of bacteria

Bacteria

Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

, predominantly certain Proteobacteria

Proteobacteria

The Proteobacteria are a major group of bacteria. They include a wide variety of pathogens, such as Escherichia, Salmonella, Vibrio, Helicobacter, and many other notable genera....

 and Actinobacteria

Actinobacteria

Actinobacteria are a group of Gram-positive bacteria with high guanine and cytosine content. They can be terrestrial or aquatic. Actinobacteria is one of the dominant phyla of the bacteria....

. The atomic-resolution 3-dimensional structure of an SAH riboswitch has been solved using X-ray crystallography.