archaeosine-tRNA biosynthesis PMID:10862614 UPa:UPA00393 PMID:12054814 PMID:16407303 Biosynthesis of archaeosine, an archaea-specific modified base. Archaeosine (7-formamidino-7-deazaguanosine) is a structural variant of the hypermodified nucleoside 7-deazaguanosine. The biosynthesis pathway starts with archaeosine tRNA-guanine transglycosylase (ArcTGT) which catalyzes the exchange of guanine at position 15 in the D-loop of archaeal tRNAs with a free 7-cyano-7-deazaguanine (preQ(0)) base. pathway tRNA modification