Queuosine is a modified nucleoside that is present in certain tRNAs in bacteria and eukaryotes. Originally identified in E. coli, queuosine was found to occupy the first anticodon position of tRNAs for histidine, aspartic acid, asparagine and tyrosine. The first anticodon position pairs with the third "wobble" position in codons, and queuosine improves accuracy of translation. Synthesis of queuosine begins with GTP. In bacteria, two classes of riboswitch are known to regulate genes that are involved in the synthesis or transport of pre-queuosine1, a precursor to queuosine: PreQ1-I riboswitches and PreQ1-II riboswitches.