tRNAs possess a high content of modified
nucleosides, which display an incredible structural variety. These modified
nucleosides are conserved in their sequence and have important roles in
tRNA functions. Most often, hypermodified
nucleosides are found in the wobble position of tRNAs, which play a direct role in maintaining translational efficiency and fidelity,
codon recognition, etc. One of such hypermodified base is
queuine, which is a base analogue of
guanine, found in the first
anticodon position of specific tRNAs (
tyrosine,
histidine,
aspartate and
asparagine tRNAs). These tRNAs of the 'Q-family' originally contain
guanine in the first position of
anticodon, which is post-transcriptionally modified with
queuine by an irreversible insertion during maturation.
Queuine is ubiquitously present throughout the living system from prokaryotes to eukaryotes, including plants. Prokaryotes can synthesize
queuine de novo by a complex biosynthetic pathway, whereas eukaryotes are unable to synthesize either the precursor or
queuine. They utilize salvage system and acquire
queuine as a nutrient factor from their diet or from intestinal microflora. The tRNAs of the Q-family are completely modified in terminally differentiated somatic cells. However, hypomodification of Q-
tRNA (
queuosine-modified
tRNA) is closely associated with cell proliferation and
malignancy. The precise mechanisms of
queuine- and Q-
tRNA-mediated action are still a mystery. Direct or indirect evidence suggests that
queuine or Q-
tRNA participates in many cellular functions, such as inhibition of cell proliferation, control of aerobic and anaerobic metabolism, bacterial virulence, etc. The role of Q-
tRNA modification in cellular machinery and the signalling pathways involved therein is the focus of this review.