Fluoride riboswitch

The fluoride riboswitch (formerly called the crcB RNA motif) is a conserved RNA structure identified by bioinformatics in a wide variety of bacteria and archaea. These RNAs were later shown to function as riboswitches that sense fluoride ions. These "fluoride riboswitches" increase expression of downstream genes when fluoride levels are elevated, and the genes are proposed to help mitigate the toxic effects of very high levels of fluoride.

Many genes are presumed to be regulated by these fluoride riboswitches. Two of the most common encode proteins that are proposed to function by removing fluoride from the cell. These proteins are CrcB proteins and a fluoride-specific subtype of chloride channels referred to as EriC^F or ClC^F. ClC^F proteins have been shown to function as fluoride-specific fluoride/proton antiporters.

The three-dimensional structure of a fluoride riboswitch has been solved at atomic resolution by X-ray crystallography.

Fluoride riboswitches are found in many organisms within the domains bacteria and archaea, indicating that many of these organisms sometimes encounter elevated levels of fluoride. Of particular interest is Streptococcus mutans, a major cause of dental caries. It has been shown that sodium fluoride has inhibited the growth rate of S. mutans using glucose as an energy and carbon source. However, it is also noteworthy that many organisms that do not encounter fluoride in the human mouth carry fluoride riboswitches or resistance genes.