Subsystem: Queuosine-Archaeosine Biosynthesis
This subsystem's description is:
Queuosine (Q) and Archaeosine (G+) are hypermodified ribonucleosides found in transfer RNA (tRNA). Q is present in the anticodon region of tRNAGUN in Eukarya and Bacteria, while G+ is found at position 15 in the D-loop of archaeal tRNA. Prokaryotes produce these 7-deazaguanosine derivatives de novo from GTP, but mammals import the free base, queuine, obtained from the diet or the intestinal flora.
GTP cyclohydrolase I, the first folate biosynthesis enzyme, is also the first enzyme in the Q/G+ pathways  and QueD, QueE and QueC catalyze the subsequent steps in both Bacteria [2, 3 ] and Archaea . In Archaea, preQ0 is inserted directly into tRNA by a tRNA-guanine transglycosylase (arcTGT, EC 184.108.40.206) [5, 6], encoded by the tgtA gene . In bacteria, preQ0 is first reduced to 7-aminomethyl-7-deazaguanine (preQ1) by QueF (EC 220.127.116.11)  before insertion in substrate tRNAs by a bacterial type TGT (bTGT, EC 18.104.22.168) encoded by the tgt gene . PreQ1 is further modified on the tRNA to Q in two subsequent enzymatic steps by QueA and QueG [9, 10].
A recently discovered ATP-independent amidinotransferase, ARChaeosine Synthase or ArcS, catalyzes the final step in the G+ pathway, the conversion of preQ0-tRNA to G+-tRNA, in Euryarchaeota . However, many Crenarchaeota known to harbor G+ lack ArcS homologs. Using comparative genomics approaches, two families that could functionally replace ArcS in these organisms were identified: 1) GAT-QueC, a two-domain family with an N-terminal glutamine amidotransferase class-II domain fused to a domain homologous to QueC, the enzyme that produces preQ0; 2) QueF-like, a family homologous to the bacterial enzyme catalyzing the reduction of preQ0 to 7-aminomethyl-7-deazaguanine .
Eukaryotic TGT enzymes (eTGT) catalyze the insertion of queuine (q) and have therefore different specificity from bTGTs that favor preQ1 over q (Fig. 2) [13, 14]. In mammals, the TGT subunit (or QTRT1) and the Qv1 subunit (or QTRTD1) form an active heterodimer even if the catalytic residues reside in the TGT subunit per se [15, 16].
1. Phillips, G., et al., Biosynthesis of 7-deazaguanosine-modified tRNA nucleosides: a new role for GTP Cyclohydrolase I. J. Bacteriol., 2008. 190(24): p. 7876-7884.
2. Reader, J.S., et al., Identification of four genes necessary for biosynthesis of the modified nucleoside queuosine. J Biol Chem, 2004. 279(8): p. 6280-5.
3. McCarty, R.M., et al., The deazapurine biosynthetic pathway revealed: in vitro enzymatic synthesis of preQ0 from guanosine 5'-triphosphate in four steps. Biochemistry, 2009. 48(18): p. 3847-3852.
4. Blaby, I.K., et al., Towards a systems approach in the genetic analysis of archaea: accelerating mutant construction and phenotypic analysis in Haloferax volcanii. Archaea, 2010. 2010: p. 426239.
5. Watanabe, M., et al., Biosynthesis of archaeosine, a novel derivative of 7-deazaguanosine specific to Archaeal tRNA, proceeds via a pathway involving base replacement of the tRNA polynucleotide chain. J Biol Chem, 1997. 272(32): p. 20146-20151.
6. Bai, Y., et al., Hypermodification of tRNA in Thermophilic archaea. Cloning, overexpression, and characterization of tRNA-guanine transglycosylase from Methanococcus jannaschii. J Biol Chem, 2000. 275(37): p. 28731-8.
7. Van Lanen, S.G., et al., From cyclohydrolase to oxidoreductase: discovery of nitrile reductase activity in a common fold. Proceedings of the National Academy of Sciences USA, 2005. 102(12): p. 4264-9.
8. Noguchi, S., et al., Isolation and characterization of an Escherichia coli mutant lacking tRNA-guanine transglycosylase. Function and biosynthesis of queuosine in tRNA. J Biol Chem, 1982. 257(11): p. 6544-50.
9. Slany, R.K., M. Bosl, and H. Kersten, Transfer and isomerization of the ribose moiety of AdoMet during the biosynthesis of queuosine tRNAs, a new unique reaction catalyzed by the QueA protein from Escherichia coli. Biochimie, 1994. 76(5): p. 389-93.
10. Miles, Z.D., et al., Discovery of epoxyqueuosine (oQ) reductase reveals parallels between halorespiration and tRNA modification. Proc Natl Acad Sci U S A, 2011. 108(18): p. 7368-7372.
11. Phillips, G., et al., Discovery and characterization of an amidotransferase involved in the modification of archaeal tRNA. J Biol Chem, 2010. 285(17): p. 12706-13.
12. Phillips, G., et al., Diversity of Archaeosine Synthesis in Crenarchaeota. ACS Chemical Biology, 2011.
13. Stengl, B., K. Reuter, and G. Klebe, Mechanism and substrate specificity of tRNA-Guanine transglycosylases (TGTs): tRNA-modifying enzymes from the three different kingdoms of life share a common catalytic mechanism. Chem. Bio. Chem., 2005. 6(11): p. 1926-1939.
14. Chen, Y.-C., et al., Evolution of eukaryal tRNA-guanine transglycosylase: insight gained from the heterocyclic substrate recognition by the wild-type and mutant human and Escherichia coli tRNA-guanine transglycosylases. Nucleic Acids Research, 2011. 39(7): p. 2834-2844.
15. Chen, Y.-C., et al., Characterization of the human tRNA-guanine transglycosylase: Confirmation of the heterodimeric subunit structure. RNA, 2010. 16(5): p. 958-968.
16. Boland, C., et al., Queuosine formation in eukaryotic tRNA occurs via a mitochondria-localized heteromeric transglycosylase. J Biol Chem, 2009. 284(27): p. 18218 - 18227.
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