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Mol Cell. 2008 Mar 28;29(6):691-702. doi: 10.1016/j.molcel.2008.01.012.

Riboswitches that sense S-adenosylhomocysteine and activate genes involved in coenzyme recycling.

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Department of Molecular, Cellular and Developmental Biology, Yale University, New Haven, CT 06520, USA.


We have identified a highly conserved RNA motif that occurs upstream of genes involved in S-adenosyl-L-methionine (SAM) recycling in many Gram-positive and Gram-negative species of bacteria. The phylogenetic distribution and the conserved structural features of representatives of this motif are indicative of riboswitch function. Riboswitches are widespread metabolite-sensing gene control elements that are typically found in the 5' untranslated regions (UTRs) of bacterial mRNAs. We experimentally verified that examples of this RNA motif specifically recognize S-adenosylhomocysteine (SAH) in protein-free in vitro assays, and confirmed that these RNAs strongly discriminate against SAM and other closely related analogs. A representative SAH motif was found to activate expression of a downstream gene in vivo when the metabolite is bound. These observations confirm that SAH motif RNAs are distinct ligand-binding aptamers for a riboswitch class that selectively binds SAH and controls genes essential for recycling expended SAM coenzymes.

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