In eubacteria, the tRNA transglycosylase (Tgt) in specific tRNAs exchanges a guanine in the anticodon for 7-aminomethyl-7-deazaguanine, which is finally converted to queuosine. The tgt gene of Escherichia coli has been mapped at 9 min on the genome, and mutant pairs containing an intact or mutated tgt allele were obtained after transduction of the tgt locus by P1 bacteriophages into a genetically defined E. coli strain (S. Noguchi, Y. Nishimura, Y. Hirota, and S. Nishimura, J. Biol. Chem. 257:6544 6550, 1982). These tgt mutants grew anerobicafly with fumarate as an electron acceptor, while nitrate or trimethylamine N-oxide could not be reduced. Furthermore, molybdate reductase activity was almost lacking and the characteristic absorption maxima, corresponding to cytochrome a, and the cytochrome d complex, were not detectable in lowtemperature reduced-minus-oxidized difference spectra in anaerobically grown cells. Transduction of the mutated tgt locus into another E. coli recipient resulted in tgt mutants without anaerobic defects. Transformation of the original tgt mutants with anfnr gene-containing plasmid reversed the anaerobic defects. Clearly, the original tgt mutants harbor a second mutation, affecting the anaerobic regulator protein Fnr. The results suggest that fnr is involved in anaerobic control of components of the cytochrome d complex and of the redox system that transfers electrons to molybdate. F' plasmids containing a fused lacI-lacZ gene with the nonsense codon UAG at different positions in the lacl part were transferred to E. coli strains with a mutated or nonmutated tgt locus but intact infnr. A twofold increase in the frequency of incorrect readthrough of the UAG codon, dependent on the codon context, was observed in the tgt mutant and is suggested to be caused by a tRNATYr with G in place of queuosine.Eseherichia coli mutants with a defined genetic background were isolated, containing or lacking the tRNA guanine transglycosylase (Tgt), and consequently containing or lacking queuosine (Q), 7-(((4,5-cis-dihydroxy-2-cyclopentene-1-yl)-amino)-methyl)-7-deazaguanosine, in tRNA (31).The transglycosylase enzyme catalyzes (in tRNAsGUN specific for Asn, Asp, His, and Tyr) the exchange of the guanine residue in the first position of the anticodon for 7-aminomethyl-7-deazaguanine, a precursor of Q (32, 33). The cyclopentenediol moiety of Q is then synthesized at the level of tRNA and involves epoxy-Q which is finally converted to Q by a cobamide-dependent enzyme system (7).Noguchi et al. (31) isolated tgt mutants by random screening from a collection of Escherichia coli K-12 mutants obtained by treatment with N-methyl-N'-nitrosoguanidine. tRNA was isolated from about 400 strains, and three mutants were found with respective tRNAs containing the anticodon GUN, where N is one of the canonical nucleosides. The defective gene, named tgt, was mapped at about 9 min on the E. coli chromosome, and the gene order was shown to be phoB tgt tsx. The tgt locus was transferred into E. coli ANLO5 (see Table 1) by P1 ba...
