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The ''crcB'' RNA motif (now called the fluoride riboswitch) 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. == Discovery of the Fluoride Riboswitch == The identity of fluoride as the riboswitch ligand was accidentally discovered when a compound contaminated with fluoride caused significant conformational changes to the non-coding ''crcB'' RNA motif during an in-line probing experiment.〔 In-line probing was used to illuminate the secondary structure of the ''crcB'' RNA motif and structural changes associated with possible binding to specific metabolites or ions. The results of the probing showed the addition of increasing fluoride ion concentrations suppressed certain regions of spontaneous RNA cleavage and heightening other regions. These nucleotide regions in the ''crcB'' RNA motif play important roles in the aptamer binding region for fluoride.〔 Upon binding fluoride ions, the fluoride riboswitch showed regulation of downstream gene transcription.〔 These downstream genes transcribe fluoride sensitive enzymes 〔 such as enolase, pyrophosphatase, the presumed fluoride exporter CrcB and a superfamily of CLC membrane proteins called Eric^F proteins. The CLC^F proteins have been shown to function as fluoride transporters against fluoride toxicity.〔 The ''eric^F'' gene is a mutant version of the chloride channel gene that is less common in bacteria than chloride-specific homologs, but is nonetheless found in the genome of Streptococcus mutans. The Eric^F protein in particular carries specific amino acids in their channels that targets fluoride anions whereas the regular Eric protein favored chloride over fluoride ions.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「CrcB RNA motif」の詳細全文を読む スポンサード リンク
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