tRNA-queuosine biosynthesis PMID:17384645 PMID:12731872 pathway GO:0008616 UPa:UPA00392 PMID:17083917 PMID:19414587 PMID:15822125 In most eubacterial and eukaryotic organisms the genetically encoded guanosine in the anticodon wobble position 34 of tRNA(His), tRNA(Tyr), tRNA(Asp) and tRNA(Asn) is replaced by the hypermodified nucleoside queuosine. The biosynthesis of queuosine commences outside of any tRNA with GTP which is converted to preQ0 (7-cyano-7-deazaguanine) in a poorly characterized pathway. Compound preQ0 is a precursor of archaeosine too. Hence, its biosynthesis is described in an independant pathway. The cyano group of preQ0 is reduced to a primary amine resulting in the formation of preQ1 (queuine). The purine scaffold of queuosine is replaced by a 7-deazapurine substituted with a dihydroxycyclopentenyl-aminomethyl moiety 7-aminomethyl-7-carbaguanine). Although no clearly defined function of queuosine has been established yet, Grimm et al [PMID:17083917] suggests a role in fine tuning of translational fidelity and speed. tRNA modification