Structure-guided discovery of the metabolite carboxy-SAM that modulates tRNA function

Members of the SAM-dependent methyltransferase superfamily are involved in the modification of wobble uridine to 5-oxacetyl uridine in Gram-negative bacteria; CmoA converts SAM to carboxy-SAM (Cx-SAM; a metabolite that was unknown previously), and CmoB uses Cx-SAM to convert 5-hydroxyuridine to 5-oxyacetyl uridine in tRNA.