Transfer ribonucleic acid guanine transglycosylase isolated from rat liver

Abstract

Transfer ribonucleic acid (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 first 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 [Okada, N., Noguchi, S. Kasai, H., Shindo-Okada, N., Ohgi, T., Goto, T., & Nishimura, S. (1979) J. Biol. Chem. 254, 3067-3073]. The Km value for queuine of the rat liver enzyme is 9.2 X 10(-7) M, much lower than the values for the bases of queuosine precursors or guanine. Thus, 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, 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 of mammalian cells. 6-Thioguanine and 8-azaguanine are also found to be good substrates.

Guanine analog-induced differentiation of human promyelocytic leukemia cells and changes in queuine modification of tRNA.

“…Since 6-thioguanine and 8-azaguanine are both inhibitors of and substrates for tRNA-guanine ribosyltransferase (12,33), we decided to investigate the potential involvement of changes in queuine modification of tRNA in the induction of cell differentiation by these compounds. Figure 2 demonstrates how the queuine content of the tRNA changed in HGPRT-deficient HL-60 cells treated with 6-thioguanine.…”

“…Recent work by Ishiguro et al (17) verified that 6-thioguanine itself was capable of inducing the differentiation of HGPRT-deficient HL-60 cells, while wild-type HL-60 cells metabolized the guanine analog to the expected cytotoxic nucleotides. Although these studies demonstrated that 6-thioguanine was the metabolic form responsible for the induction of differentiation of HGPRT-deficient HL-60 cells, the molecular basis for this induction was not established.Many purine analogs, including 6-thioguanine, have been reported to be substrates for the tRNA modification enzyme tRNA-guanine ribosyltransferase (EC 2.4.2.29) (12,33). This enzyme catalyzes the exchange of the highly modified purine analog queuine [7-([3S,4R,5S]-4,5-dihydroxy-2-cyclopent-1-en-3-ylaminomethyl)-7-deazaguanine] for the major base guanine via direct base replacement, whereby the phosphodiester backbone of the tRNA is not broken (1,11,21 obtain this factor from the sera used to supplement the cell culture medium (18,20).…”

“…Many purine analogs, including 6-thioguanine, have been reported to be substrates for the tRNA modification enzyme tRNA-guanine ribosyltransferase (EC 2.4.2.29) (12,33). This enzyme catalyzes the exchange of the highly modified purine analog queuine [7-([3S,4R,5S]-4,5-dihydroxy-2-cyclopent-1-en-3-ylaminomethyl)-7-deazaguanine] for the major base guanine via direct base replacement, whereby the phosphodiester backbone of the tRNA is not broken (1,11,21 obtain this factor from the sera used to supplement the cell culture medium (18,20).…”

Hypermodification of tRNA in Thermophilic Archaea

“…First, they are the only enzymes involved in RNA modification that catalyze the replacement of a genetically encoded base with a highly modified base. Second, there is remarkable correlation of their modified base substrate with their segregation within the primary phylogenetic domains; bacterial TGTs utilize preQ 1 , eukaryotic enzymes utilize queuine (21), and the archaeal enzymes appear to utilize preQ 0 . Finally, their origin in the different domains is associated with disparate tRNA recognition elements.…”

Dictyostelium discoideum: a useful model system to evaluate the function of queuine and of the Q‐family of tRNAs

“…In eukaryotes queuine is inserted into the tRNAs in exchange for guanine by the enzyme queuine: guanine tRNAtransglycosylase [2,3]. Experiments with germ-free mice, fed on queuine-free diet, showed that mice do not synthesize queuine de novo [4,5].…”