Thermosensitive mutants of Escherichia coli K-12 altered in the catalytic Subunit and in a Regulatory factor of the glutamy-transfer ribonucleic acid synthetase

Fusion with anticodon binding domain of GluRS is not sufficient to alter the substrate specificity of a chimeric Glu-Q-RS

Glutamyl-queuosine-tRNA(Asp) synthetase (Glu-Q-RS) is a paralog of glutamyl-tRNA synthetase (GluRS) and is found in more than forty species of proteobacteria, cyanobacteria, and actinobacteria. Glu-Q-RS shows striking structural similarity with N-terminal catalytic domain of GluRS (NGluRS) but it lacks the C-terminal anticodon binding domain (CGluRS). In spite of structural similarities, Glu-Q-RS and NGluRS differ in their functional properties. Glu-Q-RS glutamylates the Q34 nucleotide of the anticodon of tRNA(Asp) whereas NGluRS constitutes the catalytic domain of GluRS catalyzing the transfer of Glu on the acceptor end of tRNA(Glu). Since NGluRS is able to catalyze aminoacylation of only tRNA(Glu) the glutamylation capacity of tRNA(Asp) by Glu-Q-RS is surprising. To understand the substrate specificity of Glu-Q-RS we undertook a systemic approach by investigating the biophysical and biochemical properties of the NGluRS (1-301), CGluRS (314-471) and Glu-Q-RS-CGluRS, (1-298 of Glu-Q-RS fused to 314-471 from GluRS). Circular dichroism, fluorescence spectroscopy and differential scanning calorimetry analyses revealed absence of N-terminal domain (1-298 of Glu-Q-RS) and C-terminal domain (314-471 from GluRS) communication in chimera, in contrast to the native full length GluRS. The chimeric Glu-Q-RS is still able to aminoacylate tRNA(Asp) but has also the capacity to bind tRNA(Glu). However the chimeric protein is unable to aminoacylate tRNA(Glu) probably as a consequence of the lack of domain-domain communication.

A C-truncated glutamyl-tRNA synthetase specific for tRNA(Glu) is stimulated by its free complementary distal domain: mechanistic and evolutionary implications

Faithful translation of the genetic code is mainly based on the specificity of tRNA aminoacylation catalyzed by aminoacyl-tRNA synthetases. These enzymes are comprised of a catalytic core and several appended domains. Bacterial glutamyl-tRNA synthetases (GluRS) contain five structural domains, the two distal ones interacting with the anticodon arm of tRNA(Glu). Thermus thermophilus GluRS requires the presence of tRNA(Glu) to bind ATP in the proper site for glutamate activation. In order to test the role of these two distal domains in this mechanism, we characterized the in vitro properties of the C-truncated Escherichia coli GluRSs N(1-313) and N(1-362), containing domains 1-3 and 1-4, respectively, and of their N-truncated complements GluRSs C(314-471) (containing domains 4 and 5) and C(363-471) (free domain 5). These C-truncated GluRSs are soluble, aminoacylate specifically tRNA(Glu), and require the presence of tRNA(Glu) to catalyze the activation of glutamate, as does full-length GluRS(1-471). The k(cat) of tRNA glutamylation catalyzed by N(1-362) is about 2000-fold lower than that catalyzed by the full-length E. coli GluRS(1-471). The addition of free domain 5 (C(363-471)) to N(1-362) strongly stimulates this k(cat) value, indicating that covalent connectivity between N(1-362) and domain 5 is not required for GluRS activity; the hyperbolic relationship between domain 5 concentration and this stimulation indicates that these proteins and tRNA(Glu) form a productive complex with a K(d) of about 100 microM. The K(d) values of tRNA(Glu) interactions with the full-length GluRS and with the truncated GluRSs N(1-362) and free domain 5 are 0.48, 0.11, and about 1.2 microM, respectively; no interaction was detected between these two complementary truncated GluRSs. These results suggest that in the presence of these truncated GluRSs, tRNA(Glu) is positioned for efficient aminoacylation by the two following steps: first, it interacts with GluRS N(1-362) via its acceptor-TPsiC stem loop domain and then with free domain 5 via its anticodon-Dstem-biloop domain, which appeared later during evolution. On the other hand, tRNA glutamylation catalyzed by N(1-313) is not stimulated by its complement C(314-471), revealing the importance of the covalent connectivity between domains 3 and 4 for GluRS aminoacylation activity. The K(m) values of N(1-313) and N(1-362) for each of their substrates are similar to those of full-length GluRS. These C-truncated GluRSs recognize only tRNA(Glu). These results confirm the modular nature of GluRS and support the model of a "recent" fusion of domains 4 and 5 to a proto-GluRS containing the catalytic domain and able to recognize its tRNA substrate(s).

The role of the catalytic domain of E. coli GluRS in tRNAGln discrimination

Discrimination of tRNA(Gln) is an integral function of several bacterial glutamyl-tRNA synthetases (GluRS). The origin of the discrimination is thought to arise from unfavorable interactions between tRNA(Gln) and the anticodon-binding domain of GluRS. From experiments on an anticodon-binding domain truncated Escherichia coli (E. coli) GluRS (catalytic domain) and a chimeric protein, constructed from the catalytic domain of E. coli GluRS and the anticodon-binding domain of E. coli glutaminyl-tRNA synthetase (GlnRS), we show that both proteins discriminate against E. coli tRNA(Gln). Our results demonstrate that in addition to the anticodon-binding domain, tRNA(Gln) discriminatory elements may be present in the catalytic domain in E. coli GluRS as well.

A chimaeric glutamyl:glutaminyl-tRNA synthetase: implications for evolution

aaRSs (aminoacyl-tRNA synthetases) are multi-domain proteins that have evolved by domain acquisition. The anti-codon binding domain was added to the more ancient catalytic domain during aaRS evolution. Unlike in eukaryotes, the anti-codon binding domains of GluRS (glutamyl-tRNA synthetase) and GlnRS (glutaminyl-tRNA synthetase) in bacteria are structurally distinct. This originates from the unique evolutionary history of GlnRSs. Starting from the catalytic domain, eukaryotic GluRS evolved by acquiring the archaea/eukaryote-specific anti-codon binding domain after branching away from the eubacteria family. Subsequently, eukaryotic GlnRS evolved from GluRS by gene duplication and horizontally transferred to bacteria. In order to study the properties of the putative ancestral GluRS in eukaryotes, formed immediately after acquiring the anti-codon binding domain, we have designed and constructed a chimaeric protein, cGluGlnRS, consisting of the catalytic domain, Ec GluRS (Escherichia coli GluRS), and the anti-codon binding domain of EcGlnRS (E. coli GlnRS). In contrast to the isolated EcN-GluRS, cGluGlnRS showed detectable activity of glutamylation of E. coli tRNAglu and was capable of complementing an E. coli ts (temperature-sensitive)-GluRS strain at non-permissive temperatures. Both cGluGlnRS and EcN-GluRS were found to bind E. coli tRNAglu with native EcGluRS-like affinity, suggesting that the anticodon-binding domain in cGluGlnRS enhances kcat for glutamylation. This was further confirmed from similar experiments with a chimaera between EcN-GluRS and the substrate-binding domain of EcDnaK (E. coli DnaK). We also show that an extended loop, present in the anticodon-binding domains of GlnRSs, is absent in archaeal GluRS, suggesting that the loop was a later addition, generating additional anti-codon discrimination capability in GlnRS as it evolved from GluRS in eukaryotes.

An aminoacyl-tRNA synthetase-like protein encoded by the Escherichia coli yadB gene glutamylates specifically tRNAAsp

The product of the Escherichia coli yadB gene is homologous to the N-terminal part of bacterial glutamyl-tRNA synthetases (GluRSs), including the Rossmann fold with the acceptor-binding domain and the stem-contact fold. This GluRS-like protein, which lacks the anticodon-binding domain, does not use tRNA(Glu) as substrate in vitro nor in vivo, but aminoacylates tRNA(Asp) with glutamate. The yadB gene is expressed in wild-type E. coli as an operon with the dksA gene, which encodes a protein involved in the general stress response by means of its action at the translational level. The fate of the glutamylated tRNA(Asp) is not known, but its incapacity to bind elongation factor Tu suggests that it is not involved in ribosomal protein synthesis. Genes homologous to yadB are present only in bacteria, mostly in Proteobacteria. Sequence alignments and phylogenetic analyses show that the YadB proteins form a distinct monophyletic group related to the bacterial and organellar GluRSs (alpha-type GlxRSs superfamily) with ubiquitous function as suggested by the similar functional properties of the YadB homologue from Neisseria meningitidis.

Direct glutaminyl-tRNA biosynthesis and indirect asparaginyl-tRNA biosynthesis in Pseudomonas aeruginosa PAO1

The genomic sequence of Pseudomonas aeruginosa PAO1 was searched for the presence of open reading frames (ORFs) encoding enzymes potentially involved in the formation of Gln-tRNA and of Asn-tRNA. We found ORFs similar to known glutamyl-tRNA synthetases (GluRS), glutaminyl-tRNA synthetases (GlnRS), aspartyl-tRNA synthetases (AspRS), and trimeric tRNA-dependent amidotransferases (AdT) but none similar to known asparaginyl-tRNA synthetases (AsnRS). The absence of AsnRS was confirmed by biochemical tests with crude and fractionated extracts of P. aeruginosa PAO1, with the homologous tRNA as the substrate. The characterization of GluRS, AspRS, and AdT overproduced from their cloned genes in P. aeruginosa and purified to homogeneity revealed that GluRS is discriminating in the sense that it does not glutamylate tRNA(Gln), that AspRS is nondiscriminating, and that its Asp-tRNA(Asn) product is transamidated by AdT. On the other hand, tRNA(Gln) is directly glutaminylated by GlnRS. These results show that P. aeruginosa PAO1 is the first organism known to synthesize Asn-tRNA via the indirect pathway and to synthesize Gln-tRNA via the direct pathway. The essential role of AdT in the formation of Asn-tRNA in P. aeruginosa and the absence of a similar activity in the cytoplasm of eukaryotic cells identifies AdT as a potential target for antibiotics to be designed against this human pathogen. Such novel antibiotics could be active against other multidrug-resistant gram-negative pathogens such as Burkholderia and Neisseria as well as all pathogenic gram-positive bacteria.

Overproduction of the Bacillus subtilis glutamyl-tRNA synthetase in its host and its toxicity to Escherichia coli

The Bacillus subtilis glutamyl-tRNA synthetase (GluRS), encoded by the gltX gene, aminoacylates its homologous tRNAGlu and tRNAGln with glutamate. This gene was cloned with its sigmaA promoter and a downstream region including a rho-independent terminator in the shuttle vector pRB394 for Escherichia coli and B. subtilis. Transformation of B. subtilis with this recombinant plasmid (pMP411) led to a 30-fold increase of glutamyl-tRNA synthetase specific activity in crude extracts. Transformation of E. coli with this plasmid gave no recombinants, but transformation with plasmids bearing an altered gltX was successful. These results indicate that the presence of B. subtilis glutamyl-tRNA synthetase is lethal for E. coli, probably because this enzyme glutamylates tRNA1Gln in vivo as it does in vitro.Key words: glutamyl-tRNA synthetase overproduction, Bacillus subtilis, toxicity, Escherichia coli.

Analysis of the Escherichia coli genome. V. DNA sequence of the region from 76.0 to 81.5 minutes

The DNA sequence of a 225.4 kilobase segment of the Escherichia coli K-12 genome is described here, from 76.0 to 81.5 minutes on the genetic map. This brings the total of contiguous sequence from the E.coli genome project to 725.1 kb (76.0 to 92.8 minutes). We found 191 putative coding genes (ORFs) of which 72 genes were previously known, and 110 of which remain unidentified despite literature and similarity searches. Seven new genes--arsE, arsF, arsG, treF, xylR, xylG, and xylH--were identified as well as the previously mapped pit and dctA genes. The arrangement of proposed genes relative to possible promoters and terminators suggests 90 potential transcription units. Other features include 19 REP elements, 95 computer-predicted bends, 50 Chi sites, and one grey hole. Thirty-one putative signal peptides were found, including those of thirteen known membrane or periplasmic proteins. One tRNA gene (proK) and two insertion sequences (IS5 and IS150) are located in this segment. The genes in this region are organized with equal numbers oriented with or against replication.

Functions of the gene products of Escherichia coli

A list of currently identified gene products of Escherichia coli is given, together with a bibliography that provides pointers to the literature on each gene product. A scheme to categorize cellular functions is used to classify the gene products of E. coli so far identified. A count shows that the numbers of genes concerned with small-molecule metabolism are on the same order as the numbers concerned with macromolecule biosynthesis and degradation. One large category is the category of tRNAs and their synthetases. Another is the category of transport elements. The categories of cell structure and cellular processes other than metabolism are smaller. Other subjects discussed are the occurrence in the E. coli genome of redundant pairs and groups of genes of identical or closely similar function, as well as variation in the degree of density of genetic information in different parts of the genome.

Isolation of temperature-sensitive aminoacyl-tRNA synthetase mutants from an Escherichia coli strain harboring the pemK plasmid

The pem locus, which is responsible for the stable maintenance of the low copy number plasmid R100, contains the pemK gene, whose product has been shown to be a growth inhibitor. Here, we attempted to isolate mutants which became tolerant to transient induction of the PemK protein. We obtained 20 mutants (here called pkt for PemK tolerance), of which 9 were temperature sensitive for growth. We analyzed the nine mutants genetically and found that they could be classified into three complementation groups, pktA, pktB and pktC, which corresponded to three genes, ileS, gltX and asnS, encoding isoleucyl-, glutamyl- and asparaginyl-tRNA synthetases, respectively. Since these amino-acyl-tRNA synthetase mutants did not produce the PemK protein upon induction at the restrictive temperature, these mutants could be isolated because they behaved as if they were tolerant to the PemK protein. The procedure is therefore useful for isolating temperature-sensitive mutants of aminoacyl-tRNA synthetases.

Expression of the Synechocystis sp. strain PCC 6803 tRNA(Glu) gene provides tRNA for protein and chlorophyll biosynthesis

In the cyanobacterium Synechocystis sp. strain PCC 6803 (Synechocystis 6803) delta-aminolevulinic acid (ALA), the sole precursor for the synthesis of the porphyrin rings of heme and chlorophyll, is formed from glutamate activated by acylation to tRNA(Glu) (G. P. O'Neill, D. M. Peterson, A. Schön, M. W. Chen, and D. Söll, J. Bacteriol. 170:3810-3816, 1988; S. Rieble and S. I. Beale, J. Biol. Chem. 263:8864-8871, 1988). We report here that Synechocystis 6803 possesses a single tRNA(Glu) gene which was transcribed as monomeric precursor tRNA and matured into the two tRNA(Glu) species. They differed in the extent of modification of the first anticodon base, 5-methylaminomethyl-2-thiouridine (O'Neill et al., 1988). The two tRNA species had equivalent capacities to stimulate the tRNA-dependent formation of ALA in Synechocystis 6803 and to provide glutamate for protein biosynthesis in an Escherichia coli-derived translation system. These results are in support of a dual role of tRNA(Glu). The levels of tRNA(Glu) were examined by Northern (RNA) blot analysis of cellular RNA and by aminoacylation assays in cultures of Synechocystis 6803 in which the amount of chlorophyll synthesized was modulated over a 10-fold range by various illumination regimens or by the addition of inhibitors of chlorophyll and ALA biosynthesis. In these cultures, the level of tRNA(Glu) was always a constant fraction of the total tRNA population, suggesting that tRNA(Glu) and chlorophyll levels are regulated independently. In addition, the tRNA(Glu) was always fully aminoacylated in vivo.

Catabolism and nitrogen control in Escherichia coli

It would appear from these studies that nitrogen control reflects the catabolic capacity of the cell and that utilizable nitrogen sources and some carbon sources are, to some extent, in competition for this capacity. The series of catabolic events initiated by addition of D-amino acids or by growth on aldol sugars, in the presence of ammonia nitrogen in the growth medium, provide an opportunity for study of the positive aspect of nitrogen control under conditions where negative control predominates. This approach may eventually clarify the apparent interactions between the modification cascade components, PII and UT/UR, with the nitrogen regulatory gene, glnG. The utilization of nutrients by E. coli seems less a matter of energy than of expeditious use of whatever is offered in the diet. A comparison of the rate of increase of GS on cultural downshift with the rate of increase following D-glutamate addition would suggest that control by nitrogen limitation is about eight times more effective than positive activation by D-glutamate in the presence of ammonia nitrogen. This observation is consistent with the finding of an additive effect for the D-amino acids which can function as positive activators in GS regulation. It has been demonstrated for the wild-type organism that the increase in GS level generated by a mixture of D-glutamate, D-lysine, D-threonine, and glycine approximates the increase in GS level observed during step-down of the culture from an ammonia-sufficient to an ammonia-limited condition. This observation further supports the physiologic relevance of the effect of D-amino acids in nitrogen control and suggests that the apparent derepression of GS observed upon exhaustion of the ammonia nitrogen supply represents a composite of positive activation generated as alternative catabolic functions assume a greater importance. As might be expected, addition of D-glutamate to cells at the point of ammonia exhaustion had no additional positive effect. Following a downshift from glucose-ammonia-glutamine to glucose-glutamine cultural conditions, only the level of GS increases during the initial 60-minute observation period. This finding suggests that glutamine catabolism may, like D-threonine, D-lysine, and glycine, bypass the positive activation of GDH and GAT controls. The likely possibility in that the increases observed for GAT and GDH depend on D-glutamate as specific inducer. There are several instances where D-amino acids function as inducers of L-amino acid dehydrogenases and where amino acid racemase activity is directly coupled to flavoprotein dehydrogenases.(ABSTRACT TRUNCATED AT 400 WORDS)

Cloning of the gene for Escherichia coli glutamyl-tRNA synthetase

The structural gene for the glutamyl-tRNA synthetase of Escherichia coli has been cloned in E. coli strain JP1449, a thermosensitive mutant altered in this enzyme. Ampicillin-resistant and tetracycline-sensitive thermoresistant colonies were selected following the transformation of JP1449 by a bank of hybrid plasmids containing fragments from a partial Sau3A digest of chromosomal DNA inserted into the BamHI site of pBR322. One of the selected clones, HS7611, has a level of glutamyl-tRNA synthetase activity more than 20 times higher than that of a wild-type strain. The overproduced enzyme has the same molecular weight and is as thermostable as that of a wild-type strain, indicating that the complete structural gene is present in the insert. These characteristics were lost by curing this clone of its plasmid with acridine orange, and were transferred with high efficiency to the mutant strain JP1449 by transformation with the purified plasmid. A physical map of the plasmid, which contains an insert of about 2.7 kb in length, is presented.

The glutaminyl-transfer RNA synthetase of Escherichia coli. Purification, structure and function relationship

Glutaminyl-tRNA synthetase from Escherichia coli has been purified to homogeneity with a yield of about 50%. It is a monomer of about 69 000 daltons. Arginyl and glutamyl-tRNA synthetases are also monomeric synthetases of molecular weight significantly lower than 100 000. In addition it is well known that these three synthetases require their cognate tRNA to catalyze the [32P]PPi-ATP exchange. Like arginyl-tRNA synthetase, but unlike glutamyl-tRNA synthetase, glutaminyl-tRNA synthetase seems to contain some repeated sequences. Therefore no correlation can be established between the tRNA requirement of these synthetases for the catalysis of the isotope-exchange and the presence or the absence of sequence duplication. In the native enzyme four sulfhydryl groups react with dithiobisnitrobenzoic acid causing a loss of both the aminoacylation and the [32P]PPi-ATP exchange activities. The rate-limiting steps of the overall aminoacylation and its reverse reaction correspond, respectively, to the catalysis of the aminoacylation of tRNA Gln and of the the deacylation of glutaminyl-tRNA Gln. At acidic pH, glutaminyl-tRNA synthetase catalyzes the synthesis of the glutaminyl-tRNA Gln and its deacylation at significantly lower rates than the [32P]PPi-ATP exchange, indicating than glutaminyl-tRNA Gln cannot be an obligatory intermediate in this isotope exchange. These results suggest the existence of a two-step aminoacylation mechanism catalyzed by this enzyme.

The twenty aminoacyl-tRNA synthetases from Escherichia coli. General separation procedure, and comparison of the influence of pH and divalent cations on their catalytic activities

A general separation procedure of the twenty E. coli aminoacyl-tRNA synthetases including either a 105 000 g centrifugation or a poly-ethyleneglycol-dextran two-phases partition fractionation, and chromatographies on DEAE-cellulose, phosphocellulose and hydroxyapatite is described. The specific activities of the synthetases have been determined after each chromatographic step and compared to their respective activities in the 105 000 g supernatant. Some aminoacyl-tRNA synthetases were obtained at 80 per cent purity. The presence of phenylmethylsulfonyl fluoride does not significantly modify either the elution patterns of the synthetases during the various chromatographic steps or their specific activities. Thus, contrarily to enzymes from various eukaryotic organisms no significant inactivation of the E. coli aminoacyl-tRNA synthetases occurs via proteolytic processes during the purification procedure. The effects of various factors: pH, magnesium, and other bivalent cations including spermidine, were tested on the aminoacylation and the [32P] PPi-ATP isotope-exchange reactions, and the optimal aminoacylation and isotope-exchange conditions determined for 18 of the 20 E. coli aminoacyl-tRNA synthetases.

Glutamyl-gamma-methyl ester acts as a methionine analogue in Escherichia coli: analogue resistant mutants map at the metJ and metK loci

Escherichia coliK-12 mutants resistant to glutamyl-γ-methyl ester were isolated. A mutation leading to resistance of up to 1·4 mg/ml of the methionine analogue maps at min 63 and is 13% cotransducible withserAindicating an alteration in themetKgene. Another mutation leading to resistance to 3 mg/ml of the analogue and cross-resistance to other amino acid analogues maps at min 87. This mutation, which has the phenotype of MetJ−, is shown to be situated between theglpKandmetBgenes and thus indicates a different gene order from the published one.

Two modes of metabolic regulation of lysyl-transfer ribonucleic acid synthetase in Escherichia coli K-12

Lysyl-transfer ribonucleic acid (tRNA) synthetase activity was compared in three independently isolated Escherichia coli K-12 mutants of the enzyme S-adenosyl-L-methionine synthetase (metK mutants) and their isogenic parents. In all three cases the activity of the lysyl-tRNA synthetase was elevated two- to fourfold in the mutant strains. Glycyl-L-leucine (3 mM) usually enhanced lysyl-tRNA synthetase activity two- to threefold in wild-type cells but did not further stimulate the synthetase activity in metK mutants. By two other criteria, the lysyl-tRNA synthetase from wild-type cells grown with the peptide and from the metK mutant RG62, grown in minimal medium, were similar. These criteria are enhanced resistance to thermal inactivation and altered susceptibility to endogenous proteases when compared with the synthetase from wild-type cells grown in minimal medium. In a separate set of experiments, the activities of the lysyl-, arginyl-, seryl-, and valyl-tRNA synthetases were measured in an isogenic pair of relt and rel strains of E. coli grown in a relatively poor growth medium (acetate) and in enriched medium. In the rel+ strain the level of all four synthetases was higher (two- to fourfold) in the enriched medium as expected. In the rel strain the difference in the activities of the synthetases between the two media were diminished. In all four cases the activities of the synthetases were higher in acetate medium in the rel strain. Evidence is presented that these two modes of metabolic regulation act independently.

Genes for the alpha and beta subunits of the phenylalanyl-transfer ribonucleic acid synthetase of Escherichia coli

The phenylalanyl-transfer ribonucleic acid synthetase of Escherichia coli is a tetramer that contains two different kinds of polypeptide chains. To locate the genes for the two polypeptides, we analyzed temperature-sensitive mutants with defective phenylalanyl-transfer ribonucleic acid synthetases to see which subunit was altered. The method was in vitro complementation; mutant cell extracts were mixed with purified separated alpha or beta subunits of the wild-type enzyme to generate an active hybrid enzyme. With three mutants, enzyme activity appeared when alpha was added, but not when beta was added: these are, therefore, assumed to carry lesions in the gene for the alpha subunit. Two other mutants gave the opposite response and are presumably beta mutants. Enzyme activity is also generated when alpha and beta mutant extracts are mixed, but not when two alpha or two beta mutant extracts are mixed. The inactive mutant enzymes appear to be dissociated, as judged by their sedimentation in sucrose density gradients, but the dissociation may be only partial. The active enzyme generated by complementation occurred in two forms, one that resembled the native wild-type enzyme and one that sedimented more slowly. Both alpha and beta mutants are capable of generating the native form, although alpha mutants require prior urea denaturation of the defective enzyme. With the mutants thus characterized, the genes for the alpha and beta subunits (designated pheS and heT, respectively) were mapped. The gene order, as determined by transduction is aroD-pps-pheT-pheS. The pheS and pheT genes are close together and may be immediately adjacent.

Growth rate modulation of four aminoacyl-transfer ribonucleic acid synthetases in enteric bacteria

The specific activities of arginyl- glutamyl- seryl-, and valyl-transfer ribonucleic acid (tRNA) synthetases were measured in the wild-type and mutant strains of Salmonella typhimurium LT2 and Escherichia coli B/r. In media restricted only by carbon and energy source availability, the specific activities of all four enzymes were proportional to the growth rate, with the exception of seryl-tRNA synthetase in S. typhimurium, which remained essentially constant. Structural gene densities were calculated for these four enzymes and were found not to account for the variation of specific activity with growth rate.

Derepressed levels of glutamate synthase and glutamine synthetase in Escherichia coli mutants altered in glutamyl-transfer ribonucleic acid synthetase

The levels of glutamate synthase and of glutamine synthetase are both derepressed 10-fold in strain JP1449 of Escherichia coli carrying a thermosensitive mutation in the glutamyl-transfer ribonucleic acid (tRNA) synthetase and growing exponentially but at a reduced rate at a partially restrictive temperature, compared with the levels in strain AB347 isogenic with strain JP1449 except for this thermosensitive mutation and the marker aro. These two enzymes catalyze one of the two pathways for glutamate biosynthesis in E. coli, the other being defined by the glutamate dehydrogenase. We observed a correlation between the percentage of charged tRNAGlu and the level of glutamate synthase in various mutants reported to have an altered glutamyl-tRNA synthetase activity. These results suggest that a glutamyl-tRNA might be involved in the repression of the biosynthesis of the glutamate synthase and of the glutamine synthetase and would couple the regulation of the biosynthesis of these two enzymes, which can work in tandem to synthesize glutamate when the ammonia concentration is low in E. coli but whose structural genes are quite distant from each other. No derepression of the level of the glutamate dehydrogenase was observed in mutant strain JP1449 under the conditions where the levels of the glutamine synthetase and of the glutamate synthase were derepressed. This result indicates that the two pathways for glutamate biosynthesis in E. coli are under different regulatory controls. The glutamate has been reported to be probably the key regulatory element of the biosynthesis of the glutamate dehydrogenase. Our results indicate that the cell has chosen the level of glutamyl-tRNA as a more sensitive probe to regulate the biosynthesis of the enzymes of the other pathway, which must be energized at a low ammonia concentration.