SLIGHT SEQUENCE VARIATIONS OF A COMMON FOLD EXPLAIN THE SUBSTRATE SPECIFICITIES OF TRANSFER-RNA-GUANINE TRANSGLYCOSYLASES FROM THE 3 KINGDOMS

tRNA-guanine transglycosylases (TGTs) are the enzymes catalyzing the b ase exchange required for the synthesis of the modified bases derived from 7-deazaguanine in prokaryotic, archaebacterial, and eukaryotic tR NAs, Unlike the eukaryotic and archaebacterial enzymes, the prokaryoti c TGTs have been clearly identified and highly characterized both bioc hemically and structurally. The recent occurrence in sequence database s of archaebacterial and eukaryotic proteins homologous to the prokary otic TGTs reveals that all TGTs a unexpectedly adopt a common fold. Ob served sequence variations at the active site correlate well with thei r specificities for the various 7-deazaguanine derivatives and the tot al conservation of the catalytic residues strongly favors a common cat alytic mechanism for all TGTs, (C) 1997 Federation of European Biochem ical Societies.