title: PreQ1-III riboswitch
text: # PreQ1-III riboswitch
  
  PreQ1-III riboswitches are a class of riboswitches that bind $pre-queuosine_1$ $(PreQ_1),$ a precursor to the modified nucleoside queuosine. $PreQ_1-III$ riboswitches are the third class of riboswitches to be discovered that sense this ligand, and are structurally distinct from $preQ_1-I$ and $preQ_1-II$ riboswitches. Most sequenced examples of $preQ_1-III$ riboswitches are obtained from uncultivated metagenome samples, but the few examples in cultivated organisms are present in strains that are known to or suspected to be Faecalibacterium prausnitzii, a species of Gram-positive Clostridia. Known examples of $preQ_1-III$ riboswitches are found upstream of queT genes, which are expected to encode transporters of a queuosine derivative. The other two known classes of $preQ_1$ riboswitches are also commonly found upstream of queT genes.
  
  The atomic-resolution structure of a preQ1-III riboswitch has been solved by X-ray crystallography.
  
  