Transfer ribonucleic acid guanine transglycosylase isolated from rat liver

Abstract

TRNA guanine transglycosylase (guanine insertion enzyme) was isolated from rat liver and extensively purified. The enzyme catalyzes an exchange of queuine (the base of queuosine, Q) as well as its precursors and guanine for guanine originally located in the 1st position of the anticodon of undermodified tRNATyr, tRNAHis, tRNAAsn and tRNAAsp from an Escherichia coli mutant or rat ascites hepatoma cells. This is in contrast to the previous observation that E. coli tRNA-guanine transglycosylase catalyzes the exchange of queuine precursors, such as 7-(aminoethyl)-7-deazaguanine and 7-cyano-7-deazaguanine, but not of queuine itself. The Km value for queuine of the rat liver enzyme is 9.2 .times. 10-7 M, much lower than the values for the bases of queuosine precursors or guanine. The actual substrate for tRNA-guanine transglycosylase in queuosine biosynthesis in vivo in rat liver may not be 7-(aminomethyl)-7-deazaguanine, which is thought to be an actual substrate guanine in the E. coli system. Queuine or some queuine derivative may be the actual substrate for the tRNA-guanine transglycosylase reaction in the biosynthesis of Q in tRNA or mammalian cells. 6-Thioguanine and 8-azaguanine are also good substrates.