Slight sequence variations of a common fold explain the substrate specificities of tRNA‐guanine transglycosylases from the three kingdoms

tRNA-guanine transglycosylases (TGTs) are the enzymes catalyzing the base exchange required for the synthesis of the modified bases derived from 7-deazaguanine in prokaryotic, archaebacterial, and eukaryotic tRNAs. Unlike the eukaryotic and archaebacterial enzymes, the prokaryotic TGTs have been clearly identified and highly characterized both biochemically and structurally. The recent occurrence in sequence databases of archaebacterial and eukaryotic proteins homologous to the prokaryotic TGTs reveals that all TGTs unexpectedly adopt a common fold. Observed sequence variations at the active site correlate well with their specificities for the various 7-deazaguanine derivatives and the total conservation of the catalytic residues strongly favors a common catalytic mechanism for all TGTs.