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.

[3] The first anticodon position pairs with the third "wobble" position in codons, and queuosine improves accuracy of translation compared to guanosine.

[4][5][6] Synthesis of queuosine begins with GTP.

Queuosine biosynthesis genes have also been found on phage genomes and may be involved in protection from genome degradation by the host.