SAH riboswitch
Introduction


**SAH riboswitches** are a kind of [riboswitch](/wiki/Riboswitch "Riboswitch") that bind [S\-adenosylhomocysteine](/wiki/S-adenosylhomocysteine "S-adenosylhomocysteine") (SAH). When the coenzyme [S\-adenosylmethionine](/wiki/S-adenosylmethionine "S-adenosylmethionine") (SAM) is used in a [methylation](/wiki/Methylation "Methylation") reaction, SAH is produced. SAH riboswitches typically up\-regulate genes involved in recycling SAH to create more SAM (or the metabolically related [methionine](/wiki/Methionine "Methionine")). 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](/wiki/Bacteria "Bacteria"), predominantly certain [Pseudomonadota](/wiki/Pseudomonadota "Pseudomonadota") and [Actinomycetota](/wiki/Actinomycetota "Actinomycetota"). The atomic\-resolution 3\-dimensional structure of an **SAH riboswitch** has been solved using X\-ray crystallography.

[thumb\|left\|Consensus secondary structure of SAH riboswitches. Layout is similar to that used in a published depiction. Three base pairs in this secondary structure were incorrectly predicted, while an additional base pair is missing, as revealed by an atomic\-resolution tertiary structure.](/wiki/File:SAH-riboswitch.svg "SAH-riboswitch.svg")


How do SAH riboswitches work at a molecular level?