The PreQ1-I riboswitch is a cis-acting element identified in bacteria which regulates expression of genes involved in biosynthesis of the nucleoside queuosine from GTP. This RNA element known as a riboswitch binds preQ1 (pre-queuosine1), an intemediate in the queuosine pathway. Riboswitch function has been characterised in the bacterium Bacillus subtilis, where the riboswitch is located in the leader of the ykvJKLM (queCDEF) operon which encodes four genes necessary for queuosine production. In this organism, PreQ1 binding to the riboswitch aptamer is thought to induce premature transcription termination within the leader to down-regulate expression of these genes. The preQ1 riboswitch is distinguished by its unusually small aptamer, compared to other riboswitches. Its atomic-resolution three dimensional structure was determined, with the PDB ID 2L1V. PreQ1-II riboswitches also bind preQ1, but have an unrelated, distinct structure.