Archaeal tRNAs contain the modified nucleoside archaeosine at position 15. This archaeal enzyme catalyses the exchange of guanine at position 15 of tRNA with the base preQ0, which is ultimately modified to form the nucleoside archaeosine (cf. EC 18.104.22.168) .
GTP is apparently the primary precursor in the biosynthesis of queuosine, which in a series of steps is converted to 7-(cyano)-7-deazaguanine, i.e. preQ0. PreQ0 is then reduced to PreQ1, and preQ1 is subsequently inserted into the tRNA by the enzyme TGT in a transglycosylation reaction in which the genetically encoded base guanine is eliminated
the enzyme catalyzes a transglycosylation reaction in which guanine is eliminated from position 15 of the tRNA and an archaeosine precursor (preQ0) is inserted. The enzyme is able to utilize both guanine and the 7-deazaguanine base preQ0 as substrates, but not other 7-deazaguanine bases, and is able to modify tRNA from all three phylogenetic domains. The enzyme shows good activity with the tRNASer transcript as well as tRNA from Escherichia coli and yeast
Mg2+ may be responsible for conformational rigidity of the tRNA, but not for the enzymatic activity itself. Mg2+ ion is required for activity with the T7 transcript as a substrate. Activity with unfractionated Escherichia coli tRNA does not require Mg2+
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crystal structure of tRNAVal in complex with archaeosine tRNA-guanine transglycosylase at 3.3 A resolution. The enzyme binds a different tRNA conformation (lambda form), in which the canonical core is completely disrupted and the melted D arm is protruded. The PUA domain is found to be crucial for the precise location of the tRNA molecule on the enzyme
the C-terminal extension of the enzyme is not required for the selection of G15 as the site of base exchange. Truncated forms of Pyrococcus furiosus TGT retain their specificity for guanine exchange at position 15. Deletion of the PUA domain causes a 4fold drop in the observed kcat and results in a 75fold increased Km for tRNAAsp compared with full-length TGT