The PreQ1 riboswitch is a highly conserved RNA structure that is involved in regulation of the nucleoside queuosine biosynthesis in bacteria. It serves as a riboswitch that binds directly to preQ1 (pre-queuosine1 or 7-aminomethyl-7-deazaguanine), an intermediate of the queuosine pathway . Queuosine is a modified nucleoside that is present in certain tRNAs in bacteria and eukaryotes. In bacteria, queuosine is either synthesized from GTP via a committed metabolic pathway  or it is salvaged from the environment via queuosine precursor-specific transport systems .
The preQ1 riboswitch was originally identified by bioinformatics as a conserved RNA-like structure located upstream of ykvJKLM (queCDEF) operon in Bacillus subtilis and related genomes .
Further genomic searches with a revised RNA structure model identified the preQ1 riboswitches in a wide variety of bacteria including Firmicutes and Gammaproteobacteria. The preQ1 riboswitch in B. subtilis functions as a negative regulator of transcription of the queCDEF operons. PreQ1 binding to the riboswitch aptamer induces premature transcription termination within the mRNA leader of this operon and down-regulates its expression [17384645
The solved crystal structure of the preQ1 riboswitch from the queC gene from B. subtilis showed that this RNA aptamer is a unique compact pseudoknot with three loops and two stems that encapsulates the preQ1 ligand at the junction between the two stems [19285444