In nature, the riboswitch regulates the expression of genes that help the organism deal with high levels of fluoride. These genes are often pumps that allow fluoride to be exported out of the cell (Figure 4). In E. coli the gene crcB encodes a fluoride efflux channel that removes excess fluoride from the cell so that it is no longer toxic. In E. coli when the crcB gene is genetically deleted (ΔcrcB), the phenotype is increased sensitivity to fluoride and concentrations above 500μM are lethal. In our experiments we needed to utilize the ΔcrcB E. coli strain so that fluoride could accumulate intracellularly.
