Queuine (Q) is a modified nucleotide at position 34 in tRNA (queuosine is the name of the nucleoside, while queuine is the name of the nucleotide). Nucleotide modifications in tRNA are not uncommon, as tRNA is one of the most heavily modified types of RNA, and nearly 80 types of modified nucleotides have been identified. Queuosine is a very heavily modified version of guanosine (G). Modifications in tRNA have the well-known ability to control and modulate gene expression. The regulation of gene expression typically comes from some structural changes to the stem-loop structure of the tRNA. The editing that tRNA undergoes may have developed as a response to rare codons, and tRNA counteracts frameshifts by utilizing the modified bases. Other similar modifications to nucleotides impact the ability of tRNA to initiate translation, thus impeding gene expression.
This modification is particularly widespread and found amongst a variety of organisms, indicating that perhaps convergent evolution took place in the development of this nucleoside. Eukaryotic cells cannot synthesize queuosine, so they must rely on prokaryotes of the microbiome to produce and increase the availability of it within the body. Depleted levels of Q34 (queuine at position 34) are associated with the development of tumors.