BIOSYNTHESIS OF ARCHAEOSINE, A NOVEL DERIVATIVE OF 7-DEAZAGUANOSINE SPECIFIC TO ARCHAEAL TRANSFER-RNA, PROCEEDS VIA A PATHWAY INVOLVING BASE REPLACEMENT ON THE TRANSFER-RNA POLYNUCLEOTIDE CHAIN

Archaeosine is a novel derivative of 7-deazaguanosine found in transfe r RNAs of most organisms exclusively in the archaeal phylogenetic line age and is present ill the D-loop at position 15. me show that this mo dification is formed by a posttranscriptional base replacement reactio n, catalyzed by a new tRNA-guanine transglycosylase (TGT), which has b een isolated from Haloferax volcanii and purified nearly to homogeneit y. The molecular weight of the enzyme was estimated to be 78 kDa by SD S-gel electrophoresis, The enzyme can insert free 7-cyano-7-deazaguani ne (preQ(0) base) in vitro at position 15 of an H. volcanii tRNA T7 tr anscript, replacing the guanine originally located at that position wi thout breakage of the phosphodiester backbone. Since archaeosine base and 7-aminomethyl-7-deazaguanine (preQ(1) base) were not incorporated into tRNA by this enzyme, preQ(0) base appears to be the actual substr ate for the TGT of H. volcanii, a conclusion supported by characteriza tion of preQ(0) base in all acid-soluble extract of H. volcanii cells. Thus, this novel TGT in H. volcanii is a key enzyme for the biosynthe tic pathway leading to archaeosine in archaeal tRNAs.