Rare variants of the FMN riboswitch class in Clostridium difficile and other bacteria exhibit altered ligand specificity

Many bacteria use flavin mononucleotide (FMN) riboswitches to control the expression of genes responsible for the biosynthesis and transport of this enzyme cofactor or its precursor, riboflavin. Rare variants of FMN riboswitches found in strains of Clostridium difficile and some other bacteria typically control the expression of proteins annotated as transporters, including multidrug efflux pumps. These RNAs no longer recognize FMN, and differ from the original riboswitch consensus sequence at nucleotide positions normally involved in binding of the ribityl and phosphate moieties of the cofactor. Representatives of one of the two variant subtypes were found to bind the FMN precursor riboflavin and the FMN degradation products lumiflavin and lumichrome. Although the biologically relevant ligand sensed by these variant FMN riboswitches remains uncertain, our findings suggest that many strains of C. difficile might use rare riboswitches to sense flavin degradation products and activate transporters for their detoxification.