tRNA-guanine transglycosylases (TGTs) are responsible for incorporating 7-deazaguanine-modified bases into certain tRNAs in eubacteria (preQ(1)), eukarya (queuine) and archaea (preQ(0)). In each kingdom, the specific modified base is different. We have found that the eubacterial and eukaryal TGTs have evolved to be quite specific for their cognate heterocyclic base and that Cys145 (Escherichia coli) is important in recognizing the amino methyl side chain of preQ(1) (Chen et al., Nuc. Acids Res. 39 (2011) 2834 [15]). A series of mutants of the E. coli TGT have been constructed to probe the role of three other active site amino acids in the differential recognition of heterocyclic substrates. These mutants have also been used to probe the differential inhibition of E. coli versus human TGTs by pteridines. The results indicate that mutation of these active site amino acids can "open up" the active site, allowing for the binding of competitive pteridine inhibitors. However, even the "best" of these mutants still does not recognize queuine at concentrations up to 50μM, suggesting that other changes are necessary to adapt the eubacterial TGT to incorporate queuine into RNA. The pteridine inhibition results are consistent with an earlier hypothesis that pteridines may regulate eukaryal TGT activity (Jacobson et al., Nuc. Acids Res. 9 (1981) 2351 [8]).
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