Riboswitches are structured RNA domains commonly located in the 52 untranslated regions (UTRs) of messenger RNAs where they selectively bind target metabolites and regulate gene expression. We have discovered a fluoride-responsive riboswitch class which offers an unprecedented opportunity to establish the molecular basis for fluoride toxicity and its alleviation in organisms distributed among two of the three domains of life. Of particular interest is the study of fluoride riboswitches from bacterial species because of the likely implications for understanding the health effects of fluoride additives in water and dental hygiene products. We seek to provide a strong biochemical and genetic foundation for understanding how fluoride ions are sensed by many species and what genetic consequences occur when cells detect dangerous levels of this anion. Moreover, our experiments are designed to produce data that empowers other researchers who wish to conduct studies that elucidate the molecular basis of fluoride toxicity, anti-caries efficacy, and bacterial responses to fluoride exposure. We will conduct bioinformatics searches to discover additional fluoride- sensing RNAs, examine the biochemical and genetic functions of known and new-found fluoride riboswitches, and examine the function of gene products whose expression is regulated by fluoride. This knowledge will help assess the long-standing hypothesis that the antibacterial activity of fluoride contributes to the prevention of dental caries.