Regulation of the ammonia assimilatory enzymes in Salmonella typhimurium

Structural Analysis of Glutamine Synthetase from Helicobacter pylori

Glutamine synthetase (GS) is an enzyme that regulates nitrogen metabolism and synthesizes glutamine via glutamate, ATP, and ammonia. GS is a homo-oligomeric protein of eight, ten, or twelve subunits, and each subunit-subunit interface has its own active site. GS can be divided into GS I, GS II, and GS III. GS I and GS III form dodecamer in bacteria and archaea, whereas GS II form decamer in eukaryotes. GS I can be further subdivided into GS I-α and GS I-β according to its sequence and regulatory mechanism. GS is an essential protein for the survival of Helicobacter pylori which its infection could promote gastroduodenal diseases. Here, we determined the crystal structures of the GS from H. pylori (Hpy GS) in its apo- and substrate-bound forms at 2.8 Å and 2.9 Å resolution, respectively. Hpy GS formed a dodecamer composed of two hexameric rings stacked face-to-face. Hpy GS, which belongs to GS I, cannot be clearly classified as either GS I-α or GS I-β based on its sequence and regulatory mechanism. In this study, we propose that Hpy GS could be classified as a new GS-I subfamily and provide structural information on the apo- and substrate-bound forms of the protein.

Regulation of glutamate dehydrogenase expression in Pseudomonas putida results from its direct repression by NtrC under nitrogen-limiting conditions

Nitrogen-regulated genes in enterobacteria are positively controlled by the transcriptional activator of σ(N) -dependent promoters NtrC, either directly or indirectly, through the dual regulator Nac. Similar to enterobacteria, gdhA encoding glutamate dehydrogenase from Pseudomonas putida is one of the few genes that is induced by excess nitrogen. In P. putida, the binding of NtrC to the gdhA promoter region and in vitro transcription suggest that, unlike its enterobacterial homologue that is repressed by Nac, gdhA is directly repressed by NtrC. Footprinting analyses demonstrated that NtrC binds to four distinct sites in the gdhA promoter. NtrC dimers bind cooperatively, and those bound closer to the promoter interact with the dimers bound further upstream, thus producing a proposed repressor loop in the DNA. The formation of the higher-order complex and the repressor loop appears to be important for repression but not absolutely essential. Both the phosphorylated and the non-phosphorylated forms of NtrC efficiently repressed gdhA transcription in vitro and in vivo. Therefore, NtrC repression of gdhA under nitrogen-limiting conditions does not depend on the phosphorylation of the regulator; rather, it relies on an increase in the repressor concentration under these conditions.

Global transcriptional analysis of Methanosarcina mazei strain Gö1 under different nitrogen availabilities

Certain archaeal species can fix molecular nitrogen under nitrogen limiting conditions although little is known about this process at either the genetic or molecular level. To address this on a genome-wide scale, transcriptional analysis was performed on the model methanogen Methanosarcina mazei strain Gö1 using DNA-microarrays. The genomic expression patterns for cells grown under nitrogen fixing conditions versus nitrogen sufficiency (10 mM ammonium) revealed that approximately 5% of all genes are differentially expressed. Besides a small set of genes previously known to be up-regulated under nitrogen limitation, 14 additional genes involved in nitrogen metabolism were identified plus 10 genes encoding potential transcriptional regulators, 13 genes involved in carbon metabolism, 3 genes in general stress response, 8 putative transporter genes, and an additional 21 genes with unknown function. Quantitative reverse transcriptase PCR experiments confirmed the differential expression of a subset of these genes. Promoter analysis revealed a palindromic DNA motif centered nearby the transcriptional start point for several genes up-regulated under nitrogen limitation. A bioinformatics study demonstrated the presence of this motif in the up-stream region of 52 genes genome-wide, the majority of which showed nitrogen dependent differential transcription. We therefore hypothesize that this DNA element is involved in nitrogen control in M. mazei where it may act as a binding site for a regulatory protein.

Glutamate synthase: structural, mechanistic and regulatory properties, and role in the amino acid metabolism

Ammonium ion assimilation constitutes a central metabolic pathway in many organisms, and glutamate synthase, in concert with glutamine synthetase (GS, EC 6.3.1.2), plays the primary role of ammonium ion incorporation into glutamine and glutamate. Glutamate synthase occurs in three forms that can be distinguished based on whether they use NADPH (NADPH-GOGAT, EC 1.4.1.13), NADH (NADH-GOGAT, EC 1.4.1.14) or reduced ferredoxin (Fd-GOGAT, EC 1.4.7.1) as the electron donor for the (two-electron) conversion of L-glutamine plus 2-oxoglutarate to L-glutamate. The distribution of these three forms of glutamate synthase in different tissues is quite specific to the organism in question. Gene structures have been determined for Fd-, NADH- and NADPH-dependent glutamate synthases from different organisms, as shown by searches in nucleic acid sequence data banks. Fd-glutamate synthase contains two electron-carrying prosthetic groups, the redox properties of which are discussed. A description of the ferredoxin binding by Fd-glutamate synthase is also presented. In plants, including nitrogen-fixing legumes, Fd-glutamate synthase and NADH-glutamate synthase supply glutamate during the nitrogen assimilation and translocation. The biological functions of Fd-glutamate synthase and NADH-glutamate synthase, which show a highly tissue-specific distribution pattern, are tightly related to the regulation by the light and metabolite sensing systems. Analysis of mutants and transgenic studies have provided insights into the primary individual functions of Fd-glutamate synthase and NADH-glutamate synthase. These studies also provided evidence that glutamate dehydrogenase (NADH-GDH, EC 1.4.1.2) does not represent a significant alternate route for glutamate formation in plants. Taken together, biochemical analysis and genetic and molecular data imply that Fd-glutamate synthase incorporates photorespiratory and non-photorespiratory ammonium and provides nitrogen for transport to maintain nitrogen status in plants. Fd-glutamate synthase also plays a role that is redundant, in several important aspects, to that played by NADH-glutamate synthase in ammonium assimilation and nitrogen transport.

Biosynthesis of Glutamate, Aspartate, Asparagine, L-Alanine, and D-Alanine

Glutamate, aspartate, asparagine, L-alanine, and D-alanine are derived from intermediates of central metabolism, mostly the citric acid cycle, in one or two steps. While the pathways are short, the importance and complexity of the functions of these amino acids befit their proximity to central metabolism. Inorganic nitrogen (ammonia) is assimilated into glutamate, which is the major intracellular nitrogen donor. Glutamate is a precursor for arginine, glutamine, proline, and the polyamines. Glutamate degradation is also important for survival in acidic environments, and changes in glutamate concentration accompany changes in osmolarity. Aspartate is a precursor for asparagine, isoleucine, methionine, lysine, threonine, pyrimidines, NAD, and pantothenate; a nitrogen donor for arginine and purine synthesis; and an important metabolic effector controlling the interconversion of C3 and C4 intermediates and the activity of the DcuS-DcuR two-component system. Finally, L- and D-alanine are components of the peptide of peptidoglycan, and L-alanine is an effector of the leucine responsive regulatory protein and an inhibitor of glutamine synthetase (GS). This review summarizes the genes and enzymes of glutamate, aspartate, asparagine, L-alanine, and D-alanine synthesis and the regulators and environmental factors that control the expression of these genes. Glutamate dehydrogenase (GDH) deficient strains of E. coli, K. aerogenes, and S. enterica serovar Typhimurium grow normally in glucose containing (energy-rich) minimal medium but are at a competitive disadvantage in energy limited medium. Glutamate, aspartate, asparagine, L-alanine, and D-alanine have multiple transport systems.

Regulation and differential expression of gdhA encoding NADP-specific glutamate dehydrogenase in Neisseria meningitidis clinical isolates

Meningococcal gdhA, encoding the NADP-specific l-glutamate dehydrogenase (NADP-GDH), is essential for systemic infection in an infant rat model. In this paper, a limited transcriptional analysis detected differences in gdhA expression among clinical isolates. In strains expressing high levels of gdhA mRNA, two promoters, gdhA P1 and gdhA P2, initiated transcription of gdhA. In contrast, in strains expressing low mRNA levels, gdhA P2 was not active because of weak expression of gdhR, an associated regulatory gene. Gene knock-out and complementation of a gdhR-defective mutant confirmed that GdhR is a positive regulator for gdhA P2. Trans-activation of gdhA P2 was maximal in complex medium during late logarithmic growth phase and in chemical defined medium (MCDA) when glucose (MCDA-glucose) instead of lactate (MCDA-lactate) was used as a carbon source in the presence of glutamate. gdhR knock-out mutants lost both growth phase and carbon source regulation, and exhibited a growth defect more severe in MCDA-glucose than in MCDA-lactate. DNA-protein interaction studies demonstrated that 2-oxoglutarate, a product of the catabolic reaction of the NADP-GDH and an intermediate of the tricarboxylic acid (TCA) cycle, inhibits binding of GdhR to gdhA P2.

Characterization of Pseudomonas putida genes responsive to nutrient limitation

The low bioavailability of nutrients and oxygen in the soil environment has hampered successful expression of biodegradation and biocontrol genes that are driven by promoters highly active during routine laboratory conditions of high availability of nutrients and oxygen. Hence, in the present study, expression of thegus-tagged genes in 12 Tn5-gusmutants of the soil microbePseudomonas putidaPNL-MK25 were examined under various conditions chosen to mimic the soil environment: low carbon, phosphate, nitrate or oxygen, and in the rhizosphere. Based on their expression profiles, three nutrient-responsive mutant (NRM) strains, NRM5, NRM7 and NRM17, were selected for identification of the tagged genes. In strain NRM5, expression of the glutamate dehydrogenase (gdhA) gene was increased 4·9–26·4-fold under various low-nutrient conditions. In NRM7, expression of the novel NADPH : quinone oxidoreductase-like (nql) gene was consistently amongst the highest and was synergistically upregulated by low-nutrient and anoxic conditions. ThecyoDgene in NRM17, which encodes the fourth subunit of the cytochromeoubiquinol oxidase complex, had decreased expression in low-nutrient conditions but its absolute expression level was still amongst the highest. Additionally, it was independent of oxygen availability, in contrast to that inEscherichia coli.

Roles of glutamate synthase, gltBD, and gltF in nitrogen metabolism of Escherichia coli and Klebsiella aerogenes

Mutants of Escherichia coli and Klebsiella aerogenes that are deficient in glutamate synthase (glutamate-oxoglutarate amidotransferase [GOGAT]) activity have difficulty growing with nitrogen sources other than ammonia. Two models have been proposed to account for this inability to grow. One model postulated an imbalance between glutamine synthesis and glutamine degradation that led to a repression of the Ntr system and the subsequent failure to activate transcription of genes required for the use of alternative nitrogen sources. The other model postulated that mutations in gltB or gltD (which encode the subunits of GOGAT) were polar on a downstream gene, gltF, which is necessary for proper activation of gene expression by the Ntr system. The data reported here show that the gltF model is incorrect for three reasons: first, a nonpolar gltB and a polar gltD mutation of K. aerogenes both show the same phenotype; second, K. aerogenes and several other enteric bacteria lack a gene homologous to gltF; and third, mutants of E. coli whose gltF gene has been deleted show no defect in nitrogen metabolism. The argument that accumulated glutamine represses the Ntr system in gltB or gltD mutants is also incorrect, because these mutants can derepress the Ntr system normally so long as sufficient glutamate is supplied. Thus, we conclude that gltB or gltD mutants grow slowly on many poor nitrogen sources because they are starved for glutamate. Much of the glutamate formed by catabolism of alternative nitrogen sources is converted to glutamine, which cannot be efficiently converted to glutamate in the absence of GOGAT activity. Finally, GOGAT-deficient E. coli cells growing with glutamine as the sole nitrogen source increase their synthesis of the other glutamate-forming enzyme, glutamate dehydrogenase, severalfold, but this is still insufficient to allow rapid growth under these conditions.

In vivo fluxes in the ammonium-assimilatory pathways in corynebacterium glutamicum studied by 15N nuclear magnetic resonance

Glutamate dehydrogenase (GDH) and glutamine synthetase (GS)-glutamine 2-oxoglutarate-aminotransferase (GOGAT) represent the two main pathways of ammonium assimilation in Corynebacterium glutamicum. In this study, the ammonium assimilating fluxes in vivo in the wild-type ATCC 13032 strain and its GDH mutant were quantitated in continuous cultures. To do this, the incorporation of 15N label from [15N]ammonium in glutamate and glutamine was monitored with a time resolution of about 10 min with in vivo 15N nuclear magnetic resonance (NMR) used in combination with a recently developed high-cell-density membrane-cyclone NMR bioreactor system. The data were used to tune a standard differential equation model of ammonium assimilation that comprised ammonia transmembrane diffusion, GDH, GS, GOGAT, and glutamine amidotransferases, as well as the anabolic incorporation of glutamate and glutamine into biomass. The results provided a detailed picture of the fluxes involved in ammonium assimilation in the two different C. glutamicum strains in vivo. In both strains, transmembrane equilibration of 100 mM [15N]ammonium took less than 2 min. In the wild type, an unexpectedly high fraction of 28% of the NH4+ was assimilated via the GS reaction in glutamine, while 72% were assimilated by the reversible GDH reaction via glutamate. GOGAT was inactive. The analysis identified glutamine as an important nitrogen donor in amidotransferase reactions. The experimentally determined amount of 28% of nitrogen assimilated via glutamine is close to a theoretical 21% calculated from the high peptidoglycan content of C. glutamicum. In the GDH mutant, glutamate was exclusively synthesized over the GS/GOGAT pathway. Its level was threefold reduced compared to the wild type.

Transcriptional repression of gdhA in Escherichia coli is mediated by the Nac protein

In this work we show that the nac gene from Escherichia coli is transcriptionally active, and that its expression is dependent on NRI (NtrC) and sigma-54. Northern blot experiments show a monocistronic nac-specific mRNA that is detected when wild-type cells are grown in nitrogen-limiting conditions. Our data also show that in nitrogen-limiting conditions Nac is involved in the transcriptional repression of the gdhA gene (encoding glutamate dehydrogenase) except when L-glutamine is used as the only nitrogen source. Moreover, the high level of GDH activity observed in a nac mutant strain is reduced when a wild-type nac gene is introduced under control of the lac promoter in N-limiting conditions, but not in L-glutamine or N-excess. These results suggest the existence of an additional mechanism responsible for overcoming repression by Nac.

Use of an in vivo titration method to study a global regulator: effect of varying Lrp levels on expression of gltBDF in Escherichia coli

Most studies of global regulatory proteins are performed in vitro or involve phenotypic comparisons between wild-type and mutant strains. We report the use of strains in which the gene for the leucine-responsive regulatory protein (lrp) is transcribed from isopropyl-beta-D-thiogalactopyranoside (IPTG)-inducible promoters for the purpose of continuously varying the in vivo concentration of Lrp. To obtain a broad range of Lrp concentrations, strains were employed that contained the lrp fusion either in the chromosome (I. C. Blomfield, P. J. Calie, K. J. Eberhardt, M. S. McClain, and B. I. Eisenstein, J. Bacteriol. 175:27-36, 1993) or on a multicopy plasmid. Western blot (immunoblot) analysis with polyclonal antiserum to Lrp confirmed that Lrp levels could be varied more than 70-fold by growing the strains in glucose minimal 3-(N-morpholino)propanesulfonic acid (MOPS) medium containing different amounts of IPTG. Expression of an Lrp-regulated gltB::lacZ operon fusion was measured over this range of Lrp concentrations. beta-Galactosidase activity rose with increasing Lrp levels up to the level of Lrp found in wild-type strains, at which point expression is maximal. The presence of leucine in the medium increased the level of Lrp necessary to achieve half-maximal expression of the gltB::lacZ fusion, as predicted by earlier in vitro studies (B. R. Ernsting, J. W. Denninger, R. M. Blumenthal, and R. G. Matthews, J. Bacteriol. 175:7160-7169, 1993). Interestingly, levels of Lrp greater than those in wild-type cells interfered with activation of gltB::lacZ expression. The growth rate of cultures correlated with the intracellular Lrp concentration: levels of Lrp either lower or higher than wild-type levels resulted in significantly slower growth rates. Thus, the level of Lrp in the cell appears to be optimal for rapid growth in minimal medium, and the gltBDF control region is designed to give maximal expression at this Lrp level.

The leucine-responsive regulatory protein, a global regulator of metabolism in Escherichia coli

The leucine-responsive regulatory protein (Lrp) regulates the expression of more than 40 genes and proteins in Escherichia coli. Among the operons that are positively regulated by Lrp are operons involved in amino acid biosynthesis (ilvIH, serA)), in the biosynthesis of pili (pap, fan, fim), and in the assimilation of ammonia (glnA, gltBD). Negatively regulated operons include operons involved in amino acid catabolism (sdaA, tdh) and peptide transport (opp) and the operon coding for Lrp itself (lrp). Detailed studies of a few members of the regulon have shown that Lrp can act directly to activate or repress transcription of target operons. A substantial fraction of operons regulated by Lrp are also regulated by leucine, and the effect of leucine on expression of these operons requires a functional Lrp protein. The patterns of regulation are surprising and interesting: in some cases activation or repression mediated by Lrp is antagonized by leucine, in other cases Lrp-mediated activation or repression is potentiated by leucine, and in still other cases leucine has no effect on Lrp-mediated regulation. Current research is just beginning to elucidate the detailed mechanisms by which Lrp can mediate such a broad spectrum of regulatory effects. Our view of the role of Lrp in metabolism may change as more members of the regulon are identified and their regulation characterized, but at this point Lrp seems to be important in regulating nitrogen metabolism and one-carbon metabolism, permitting adaptations to feast and to famine.

Accumulation of glutamate by Salmonella typhimurium in response to osmotic stress

Salmonella typhimurium accumulates glutamate in response to osmotic stress. Cells in aerobic exponential growth have an intracellular pool of approximately 125 nmol of glutamate mg of protein-1. When cells were grown in minimal medium with 500 mM NaCl, KCl, or sucrose, 290 to 430 nmol of glutamate was found to accumulate. Values were lower when cells were harvested in stationary phase. Cells were grown in conventional medium, harvested, washed, resuspended in the control medium or in medium with osmolytes, and aerated for 1 h. With aeration, glutamate was found to accumulate at levels comparable to those observed in exponential cultures. Antibiotics inhibiting protein synthesis did not affect glutamate accumulation when cells were aerated. Strains with mutations in glutamate synthase (glt) or in glutamate dehydrogenase (gdh) accumulated nearly normal levels of glutamate under these conditions. A double (gdh glt) mutant accumulated much less glutamate (63.9 nmol mg of protein-1), but a 1.9-fold excess accumulated when cells were aerated with osmotic stress. Methionine sulfone, an inhibitor of glutamate synthase, did not prevent accumulation of glutamate in cells aerated with osmotic stress. Glutamate dehydrogenase is thought to have minimum activity when ammonium is limiting. Resuspending cells with limiting ammonium reduced glutamate production but did not eliminate accumulation of excess glutamate when cells were osmotically stressed. Amino oxyacetic acid, an inhibitor of transamination reactions, did not prevent accumulation of excess glutamate.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification and properties of NADP-dependent glutamate dehydrogenase from Ruminococcus flavefaciens FD-1

Glutamate dehydrogenase (GDH) (L-glutamate:NADP+ oxidoreductase, deaminating, EC 1.4.1.4) from the cellulolytic ruminal bacterium Ruminococcus flavefaciens has been purified and characterized. The native enzyme and subunit are 280 and 48 kDa, respectively, suggesting that the native enzyme is a hexamer. The enzyme requires 0.5 M KCl for optimal activity and has a pH optimum of 6.9 to 7.0. The Kms for ammonia, alpha-ketoglutarate, and glutamate are 19, 0.41, and 62 mM, respectively. The sigmoidal NADPH saturation curve revealed positive cooperativity for the binding of this coenzyme. The first residue in the N-terminal amino acid sequence from R. flavefaciens GDH was alanine, suggesting that the protein may be modified posttranslationally. Comparison of the N-terminal sequence with those of Escherichia coli, Salmonella typhimurium, and Clostridium symbiosum revealed only 39% amino acid homologies. The GDH from R. flavefaciens was unique in that its specific activity was highest during ammonia-limited growth but was not affected by ammonia shock treatment (20 mM).

Molecular analysis of the Corynebacterium glutamicum gdh gene encoding glutamate dehydrogenase

The Corynebacterium glutamicum gdh gene encoding NADP-dependent glutamate dehydrogenase (GDH) has been isolated by complementation of the Escherichia coli gdh mutant PA340. The gdh gene was subcloned into the E. coli/C. glutamicum shuttle vector pEK0 and introduced into C. glutamicum. Recombinant strains showed approximately eightfold higher specific GDH activity (15U mg protein-1) relative to the wild type (1.8U mg protein-1). Physiological studies with wild-type and recombinant C. glutamicum strains revealed no indication of significant regulation of gdh expression. The DNA sequence of 2082 bp, including the gdh gene, 5'-, and 3'-flanking regions, was determined. The structural gene consists of 1344 bp and codes for a polypeptide of 448 amino acid residues (Mr 49,152) showing up to 53.6% identity with reported amino acid sequences of glutamate dehydrogenases from other organisms. Northern blot hybridization revealed a 1.65kb mRNA transcript, indicating that the gdh gene of C. glutamicum is monocistronic. Transcription occurred from a G residue located 284 bp upstream of the AUG considered to be the translational initiation codon.

Mutations affecting the Shine-Dalgarno sequences of the untranslated region of the Escherichia coli gltBDF operon

Individual mutations which affected each of the two Shine-Dalgarno sequences at the 5' untranslated region of the gltB gene of Escherichia coli were characterized. They were isolated in plasmids carrying a gltB'-'lacZ protein fusion preceded by the regulatory region of the gltBDF operon. Subcloning and nucleotide sequencing of approximately 1,206 bp of DNA encompassing the gltBDF regulatory region showed that the mutations affected the first base at each of the two identical Shine-Dalgarno sequences, SD1 and SD2, located 40 and 8 bases, respectively, upstream from the putative gltB open reading frame. Only mutation gltB2r227, an adenine in place of a guanine, affecting the first base of SD2, lowered beta-galactosidase expression significantly, i.e., about fivefold. The results suggest that SD2 is the preferred functional site at which ribosomes initiate gltB mRNA translation.

Protein phosphorylation and regulation of adaptive responses in bacteria

Bacteria continuously adapt to changes in their environment. Responses are largely controlled by signal transduction systems that contain two central enzymatic components, a protein kinase that uses adenosine triphosphate to phosphorylate itself at a histidine residue and a response regulator that accepts phosphoryl groups from the kinase. This conserved phosphotransfer chemistry is found in a wide range of bacterial species and operates in diverse systems to provide different regulatory outputs. The histidine kinases are frequently membrane receptor proteins that respond to environmental signals and phosphorylate response regulators that control transcription. Four specific regulatory systems are discussed in detail: chemotaxis in response to attractant and repellent stimuli (Che), regulation of gene expression in response to nitrogen deprivation (Ntr), control of the expression of enzymes and transport systems that assimilate phosphorus (Pho), and regulation of outer membrane porin expression in response to osmolarity and other culture conditions (Omp). Several additional systems are also examined, including systems that control complex developmental processes such as sporulation and fruiting-body formation, systems required for virulent infections of plant or animal host tissues, and systems that regulate transport and metabolism. Finally, an attempt is made to understand how cross-talk between parallel phosphotransfer pathways can provide a global regulatory curcuitry.

Positive regulation of glutamate biosynthesis in Bacillus subtilis

Nitrogen source regulation of glutamate synthase activity in Bacillus subtilis occurs at the level of transcription of the gltA and gltB genes, which encode the two subunits of the enzyme. We show here that transcription of gltA requires the product of gltC, a gene whose transcription is divergent from that of gltA and whose transcriptional control sequences overlap those of gltA. gltC mutants had decreased, aberrantly regulated levels of glutamate synthase activity and decreased gltA mRNA. The gltC gene product could act in trans to complement both these defects. In addition, the gltC gene product repressed its own transcription. The DNA sequence of gltC revealed that its putative product is very similar to a number of positive regulatory proteins from gram-negative bacteria (the LysR family).

Identification of a functional promoter for the Escherichia coli gdhA gene and its regulation

Glutamate dehydrogenase (GDH) catalyzes the synthesis of L-glutamate from 2-oxoglutarate and ammonia. The complete nucleotide sequence of the Escherichia coli gdhA gene, as well as its 5' and 3' flanking regions have been previously reported [Valle et al., Gene 23 (1983) 199-209; 27 (1984) 193-199]. In this paper we present data on the GDH specific activities using both excess and limiting concentrations of ammonia as nitrogen sources. Evidence is presented on the regulation of the mRNA levels for this enzyme by the ammonia concentration in the growth medium. We have identified a single and apparently invariant transcript for several metabolic growth conditions. We also report the identification of a functional promoter and the corresponding transcription start point under several growth conditions. Finally, possible regulatory sequences located at the 5' flanking region of the gdhA gene are discussed.

gltBDF operon of Escherichia coli

A 2.0-kilobase DNA fragment carrying antibiotic resistance markers was inserted into the gltB gene of Escherichia coli previously cloned in a multicopy plasmid. Replacement of the chromosomal gltB+ gene by the gltB225::omega mutation led to cells unable to synthesize glutamate synthase, utilize growth rate-limiting nitrogen sources, or derepress their glutamine synthetase. The existence of a gltBDF operon encoding the large (gltB) and small (gltD) subunits of glutamate synthase and a regulatory peptide (gltF) at 69 min of the E. coli linkage map was deduced from complementation analysis. A plasmid carrying the entire gltB+D+F+ operon complemented cells for all three of the mutant phenotypes associated with the polar gltB225::omega mutation in the chromosome. By contrast, plasmids carrying gltB+ only complemented cells for glutamate synthase activity. A major tricistronic mRNA molecule was detected from Northern (RNA blot) DNA-RNA hybridization experiments with DNA probes containing single genes of the operon. A 30,200-dalton polypeptide was identified as the gltF product, the lack of which was responsible for the inability of cells to use nitrogen-limiting sources associated with gltB225::omega.

An NAD+-dependent alanine dehydrogenase from a methylotrophic bacterium

A study was made of the NAD+-dependent alanine dehydrogenase (EC 1.4.1.1) elaborated by the methylotrophic bacterium Pseudomonas sp. strain MA when growing on succinate and NH4Cl. This enzyme was purified 400-fold and was found to be highly specific for NH3 and NAD+; however, hydroxypyruvate and bromopyruvate, but not alpha-oxoglutarate or glyoxylate, could replace pyruvate to a limited extent. The Mr of the native enzyme was shown to be 217,000, and electrophoresis in SDS/polyacrylamide gels revealed a minimum Mr of 53,000, suggesting a four-subunit structure. The enzyme, which has a pH optimum of 9.0, operated almost exclusively in the aminating direction in vitro. It was induced by NH3 or by alanine, and was repressed by growth on methylamine or glutamate. It is suggested that this enzyme has two roles in this organism, namely in NH3 assimilation and in alanine catabolism.

Fine structure analysis of Salmonella typhimurium glutamate synthase genes

Glutamate synthase activity is required for the growth of Salmonella typhimurium on media containing a growth-rate-limiting nitrogen source. Mutations that alter glutamate synthase activity had been identified in the gltB gene, but it was not known which of the two nonidentical subunits of the enzyme was altered. To examine the gene-protein relationship of the glt region, two nonsense mutations were identified and used to demonstrate that gltB encodes the large subunit of the enzyme. Six strains with independent Mu cts d1 (lac bla) insertions were isolated, from which a collection of deletion mutations was obtained. The deletions were transduced with the nonsense mutations and 38 other glt point mutations to construct a fine-structure genetic map. Chromosome mobilization studies, mediated by Hfr derivatives of Mu cts d1 lysogens, showed that gltB is transcribed in a clockwise direction, as shown in the S. typhimurium linkage map. Studies of the polar effects of three Mu cts d1 insertions indicated that the gene for the small subunit maps clockwise to gltB and that the two genes are cotranscribed to form a glt operon.

Characterization of mutations that lie in the promoter-regulatory region for glnA, the structural gene encoding glutamine synthetase

In enteric bacteria products of nitrogen regulatory genes ntrA, ntrB and ntrC are known to regulate transcription both positively and negatively at glnA, the structural gene encoding glutamine synthetase [L-glutamate:ammonia-ligase (ADP-forming), EC 6.3.1.2]. We have characterized two types of cis-acting mutations in the glnA promoter-regulatory region. One type, which we have called promoter Up [glnAp (Up)], elevates transcription of glnA to high levels without need for ntr-mediated activation but leaves expression sensitive to ntr-mediated repression. The other type renders glnA transcription insensitive to repression but leaves it normally responsive to activation. Properties of the two types of promoter-regulatory mutations suggest that sites for ntr-mediated activation of glnA transcription are functionally distinct from sites for ntr-mediated repression.

Cloning and characterization of gdhA, the structural gene for glutamate dehydrogenase of Salmonella typhimurium

Glutamic acid is synthesized in enteric bacteria by either glutamate dehydrogenase or by the coupled activities of glutamate synthase and glutamine synthetase. A hybrid plasmid containing a fragment of the Salmonella typhimurium chromosome cloned into pBR328 restores growth of glutamate auxotrophs of S. typhimurium and Escherichia coli strains which have mutations in the genes for glutamate dehydrogenase and glutamate synthase. A 2.2-kilobase pair region was shown by complementation analysis, enzyme activity measurements, and the maxicell protein synthesizing system to carry the entire glutamate dehydrogenase structural gene, gdhA. Glutamate dehydrogenase encoded by gdhA carried on recombinant plasmids was elevated 5- to over 100-fold in S. typhimurium or E. coli cells and was regulated in both organisms. The gdhA promoter was located by recombination studies and by the in vitro fusion to, and activation of, a promoter-deficient galK gene. Additionally, S. typhimurium gdhA DNA was shown to hybridize to single restriction fragments of chromosomes from other enteric bacteria and from Saccharomyces cerevisiae.

Regulation of transcription of glnA, the structural gene encoding glutamine synthetase, in glnA::Mu d1 (ApR, lac) fusion strains of Salmonella typhimurium

Using the Casadaban Mu d1 phage (Casadaban and Cohen 1979) we fused cis-acting regulatory sites for the Salmonella typhimurium glnA gene, the structural gene encoding glutamine synthetase, to lacZ so that transcription of lacZ was controlled by the glnA promoter-operator. Activities of beta-galactosidase in two glnA::Mu d1 fusion strains were high, approximately 25% and 125% the induced level of beta-galactosidase when transcription of lacZ is under control of the lac promoter, indicating that glutamine synthetase is not required to activate transcription of its own structural gene. Introduction of nitrogen regulatory mutations ntrA::Tn10 or ntrC::Tn10 into fusion strains resulted in greatly decreased synthesis of beta-galactosidase indicating that the positive regulatory factors encoded by ntrA and ntrC activate glnA expression at the level of transcription. Comparison of beta-galactosidase activities in fusion strains with those in fusions carrying ntrC or ntrA mutations indicated that: 1) the magnitude of activation of glnA expression is at least 43-fold; 2) the magnitude of repression is approximately 13-fold and repression occurs at the level of transcription; 3) the degree of modulation of glnA expression by ntr products is at least 560-fold (13 X 43); and 4) glutamine synthetase is not required for repression of transcription of its own structural gene. In contrast to strains carrying non-polar mutations in glnA, strains carrying glnA insertion mutations, including glnA::Mu d1 fusions, are apparently defective in activating expression of some nitrogen controlled genes other than glnA. Defects cannot be accounted for by the absence of glutamine synthetase protein or catalytic activity; they appear to be due to decreased expression of nitrogen regulatory genes ntrB and/or ntrC, which are adjacent to glnA.

Overproduction of nitrogenase by nitrogen-limited cultures of Rhodopseudomonas palustris

Rhodopseudomonas palustris cells grown on limiting nitrogen produced four- to eightfold higher nitrogenase specific activity relative to cells sparged with N2. The high activity of N-limited cells was the result of overproduction of the nitrogenase proteins. This was shown by four independent techniques: (i) titration of the Mo-Fe protein in cell-free extracts with Fe protein from Azotobacter vinelandii; (ii) direct detection of the subunits of Mo-Fe protein by sodium dodecyl sulfate-polyacrylamide gel electrophoresis; (iii) monitoring of the electron paramagnetic resonance spectrum of Mo-Fe protein in whole cells; and (iv) immunological assay of the Fe protein level with an antiserum against the homologous protein of Rhodospirillum rubrum. The derepressed level of nitrogenase found in N2-grown cells was not due to an increased turnover of nitrogenase. The apparent half-lives of nitrogenase in N2-grown and N-limited cells were 58 and 98 h, respectively, but were too long to account for the difference in enzyme level. Half-lives were determined by measuring nitrogenase after repression of de novo synthesis by ammonia and subsequent release of nitrogenase switch-off by methionine sulfoximine. Observations were extended to R. rubrum, Rhodopseudomonas capsulata, and Rhodomicrobium vannielii and indicated that overproduction of nitrogenase under nitrogen limitation is not an exceptional property of R. palustris, but rather a general property of phototrophic bacteria.

The regulation of the ammonia assimilatory enzymes in Rel+ and Rel- strains of Salmonella typhimurium

The influence of the relA1 mutation on the regulation of the ammonia assimilatory enzymes, glutamate dehydrogenase (EC 1.4.1.4), glutamine synthetase (EC 6.3.1.2), and glutamate synthase (EC 1.4.1.3), was examined. When cells grown in rich media (either Luria broth or glucose-ammonia plus casamino acids) were transferred to a glucose-ammonia medium, the relA mutant failed to resume growth and did not have the same increase in any of the assimilatory enzyme activities as the rel+ strain. This effect was particularly dramatic for glutamate dehydrogenase, which increased 6-fold in the rel+ strain. Measurements of the guanosine nucleotide concentrations showed that the rel+ strain had a ppGpp concentration about 9 times that of the relA mutant 5 min after the shift to minimal medium. These results are consistent with those for other biosynthetic enzymes and show that the ammonia assimilatory enzymes require a relA product for their synthesis during shift from rich to minimal media. In addition, we examined the response of these strains to a change in nitrogen source. The relA mutant again failed to resume growth after a shift from glucose-ammonia to glucose-arginine medium. Even though the ppGpp concentration did not increase, the rel+ strain grew and increased glutamine synthetase activities about 2-fold. These changes the absence of increased ppGpp levels suggest that some other relA-mediated function is important during this change in nitrogen source.

Genetic characterization of the glutamate dehydrogenase gene (gdhA) of Salmonella typhimurium

Salmonella typhimurium mutants, either devoid or glutamate dehydrogenase activity or having a thermolabile glutamate dehydrogenase protein, were used to identify the structural gene (gdhA) for this enzyme. Transductions showed that the mutations producing these phenotypes were linked to both the pncA and nit genes, placing the gdhA locus between 23 and 30 U on the S. typhimurium chromosome. Additional transductions with several Tn10 insertions established the gene order as pncA-gdhA-nit. Since few genetic markers exist in this region of the chromosome, Hfr strains were constructed to orient the pncA-gdhA-nit cluster with outside genes. Conjugation experiments provided evidence for the gene order pyrD-pncA-gdhA-nit-trp. To further characterize gdhA, we used Mu cts d1 (Apr lac) insertions in this gene to select numerous strains containing deletions with various endpoints. Transductions of these deletions with strains containing different gdh mutations and with a mutant having a thermolabile glutamate dehydrogenase protein permitted us to construct a deletion map of the gdhA region.

Identification of the structural genes for glutamate synthase and genetic characterization of this region of the Salmonella typhimurium chromosome

Salmonella typhimurium cells require glutamate synthase activity for growth in media containing a growth rate-limiting nitrogen source. Although this enzyme plays a critical role in ammonia assimilation, little is known about the organization and regulation of the structural genes for its two subunits. To identify the location of the structural genes, mutants having heat-labile glutamate synthase activities were isolated and characterized. Mutations that altered glutamate synthase activity were mapped at 69 U on the S. typhimurium chromosome. Four strains with independent Tn10 insertions in this region were constructed and used for mutant selection and for positioning mutations affecting glutamate synthase activity relative to other genetic markers. In contrast to results obtained with Escherichia coli mutants, there was no linkage between mutations affecting glutamate synthase activity and the argG gene. The results of a combination of transduction experiments demonstrated the gene order argG-glnF-gltB-cod-argR-envB-aroE for S-typhimurium.

Regulation of glutamate dehydrogenase in Bacillus subtilis

The activity of the nicotinamide adenine dinucleotide-dependent glutamate dehydrogenase in Bacillus subtilis was influenced by the carbon source, but not the nitrogen source, in the growth medium. The highest specific activity for this enzyme was found when B. subtilis was grown in a minimal or rich medium that contained glutamate as the carbon source. It is proposed that glutamate dehydrogenase serves a catabolic function in the metabolism of glutamate, is induced by glutamate, and is subject to catabolite repression.

Glutamine auxotrophs with mutations in a nitrogen regulatory gene, ntrC, that is near glnA

Some mutations to glutamine auxotrophy in the 86 unit region of the Salmonella chromosome lie within the nitrogen regulatory gene, ntrC, rather than the structural gene encoding glutamine synthetase, glnA, Assignment of mutations to ntrC is based on fine structure mapping by P22-mediated transduction and on complementation analysis. Strains with ntrC lesions that cause glutamine auxotrophy (NtrCrepressor) have very low levels of glutamine synthetase (lower than those of strains that completely lack ntrC function and comparable to those of strains that lack ntrA function). NtrCrep strains fail to increase synthesis of glutamine synthetase or several amino acid transport components under nitrogen limiting conditions. Thus, like ntrA strains, they appear to repress glnA transcription and fail to activate transcription of glnA or other nitrogen controlled genes. Mutations that suppress the glutamine requirement caused by NtrCrep lesions arise at high frequency; these mutations also suppress the glutamine requirement caused by ntrA lesions. Several suppressor mutations result in loss of function of ntrC.

Coregulation of oxidized nicotinamide adenine dinucleotide (phosphate) transhydrogenase and glutamate dehydrogenase activities in enteric bacteria during nitrogen limitation

The relationship between oxidized nicotinamide adenine dinucleotide (phosphate) [NAD(P)+] transhydrogenase (EC 1.6.1.1) and NAD(P)+ glutamate dehydrogenase in several enteric bacteria which differ slightly in their regulation of nitrogen metabolism was studied. Escherichia coli strain K-12 was grown on glucose and various concentrations of NH4Cl as the sole nitrogen source. In the range of 0.5 to 20 mM NH4Cl, the energy-independent transhydrogenase increased two to threefold. Comparable changes occurred in NAD(P)+-linked glutamate dehydrogenase. NH4Cl concentrations of 20 to 60 mM resulted in relatively constant specific activities for both enzymes. Higher exogenous NH4Cl, however, led to a decline in both activities. Isocitrate dehydrogenase, another potential source of cellular NADPH, was insensitive to NH4Cl limitation. Similar studies in the presence of glutamate and different exogenous NH4Cl concentrations again showed concerted effects on both enzymes. Growth on glutamate as the sole nitrogen source led to severe repression of both transhydrogenase and glutamate dehydrogenase. In Salmonella typhimurium, both enzymes were unaffected by limiting NH4Cl or growth on glutamate as the sole nitrogen source. Both were, however, repressed by growth on aspartate, a potential source of cellular glutamate. Coordinate changes in glutamate dehydrogenase and transhydrogenase were also evident in Klebsiella aerogenes, particularly under conditions in which glutamate dehydrogenase was regulated inversely to glutamate synthetase. Coordinate changes in glutamate dehydrogenase and transhydrogenase in enteric bacteria are discussed in terms of the possible involvement of the latter enzyme as a direct source of NADPH in the ammonia assimilation system.

Effect of glutamine on enzymes of nitrogen metabolism in Bacillus subtilis

An earlier study of the regulation of glutamate synthase (GOGAT) in Bacillus subtilis (Deshpande et al., Bichem. Biophys. Res. Commun. 95:55--60, 1980) revealed an inverse relationship between the specific activity of this essential ammonia-assimilatory enzyme and the intracellular pool of glutamine: GOGAT activity decreased when the internal glutamine concentration reached or exceeded 2.5 mM. This finding prompted the present investigation of the intracellular events linking glutamine formation to the regulation of GOGAT. A growing culture of B. subtilis was shifted from glutamate plus NH+4 medium (high GOGAT activity) to glutamate medium (low GOGAT activity). At various times after the shift, the intracellular concentrations of aspartate, glutamate, glutamine, alanine, and NH+4 and the activities of GOGAT and glutamine synthetase (GS) were measured. After 30 min, the only significant pool level change was an eightfold increase in glutamine, which paralleled a 2- to 3-fold increase in GS activity. Approximately 15 min after the glutamine pool reached its peak, GOGAT activity began to decrease and eventually declined 2.5-fold. In contrast, when B. subtilis was shifted from glutamate medium to glutamate plus NH+4 medium, there was a 1- to 2-h lag before the glutamine pool and GS activity approached a steady state. As a result, GOGAT activity was low until the concentration of glutamine dropped below 2.5 mM. We propose that glutamine is an important regulatory element in the control of GOGAT activity and that one form of GOGAT regulation involves enzyme inactivation. In addition, these results indicate that glutamine is neither a corepressor nor a feedback inhibitor of GS.

Temperature-sensitive glutamate dehydrogenase mutants of Salmonella typhimurium

Mutants of Salmonella typhimurium defective in glutamate dehydrogenase activity were isolated in parent strains lacking glutamate synthase activity by localizcd mutagenesis or by a general mutagenesis combined with a cycloserine enrichment for glutamate auxotrophs. Two mutants with temperature-sensitive phenotypes had glutamate dehydrogenase activities that were more thermolabile than that of an isogenic control strain. Eight other mutants had less than 10% of the wild-type glutamate dehydrogenase activity. All the mutations were cotransducible with a Tn10 element (zed-2:Tn10) located at approximately 23 U on the S. typhimurium linkage map. These data strongly indicate that this region contains the structural gene (gdhA) for glutamate dehydrogenase.

Characterization of a HindIII-generated DNA fragment carrying the glutamine synthetase gene of Salmonella typhimurium

The glnA gene, encoding glutamine synthetase in Salmonella typhimurium, has been cloned into the plasmid pBR322. One hybrid plasmid, pJB1, containing an 8.5 kb insert generated by a HindIII digest, was analyzed using eleven different restriction enzymes. Evidence that the region controlling glutamine synthetase expression remained on the insert was obtained by showing that the regulation is normal in cells carrying plasmids with the insert in the original and reversed orientation. Several new plasmids derived from pJB1 following SalI and EcoRI digestions were examined for their ability to complement a glnA202 mutation in order to locate the DNA segment needed for glutamine synthetase expression. The results show that cells containing plasmid pJB8, which has a 21 kb deletion, produce and regulate glutamine synthetase normally, whereas cells with a plasmid (pJB11) similar to pJB8, but lacking a 0.25 kb EcoRI fragment, do not exhibit glutamine synthetase activity. The analysis of proteins produced in minicells containing pJB8 and pJB11 show that they both produce a protein that migrates with the glutamine synthetase subunit. Because pJB11 makes an inactive protein of similar size to the glutamine synthetase subunit, the 0.25 kb deletion may encode only the C-terminus of this protein. Consistent with this finding is the presence of a strong RNA polymerase-binding site on pJB8 to the right of the 0.25 kb EcoRI that could correspond to a promoter near the N-terminus of the glnA gene.

Ammonium assimilation in Rhizobium meliloti

We have characterized a mutant of Rhizobium meliloti strain 2011 which cannot use ammonium as a nitrogen source. This mutant, RTm2620, was found to have significantly altered glutamate synthase activity. Both the mutant and the wild-type strains had glutamate dehydrogenase activity, which, although stimulated in the presence of glutamate and ammonium, was apparently insufficient to allow ammonium assimmilation. We conclude that the glutamine synthetase-glutamate synthase pathway may be the normal mode of ammonium assimilation by this strain in the free-living state. Independent revertants of Rm2620 were isolated and fell into two classes. Class I revertants regained partial glutamate synthase activity and had the same levels of glutamate dehydrogenase activity as Rm2620. Class II revertants retained the altered glutamate synthase activity but acquired a very high level of assimilatory glutamate dehydrogenase activity. Both classes were found to be altered in their symbiotic properties, although the original Rm2620 mutant was normal in this regard.

The enzymes of the ammonia assimilation in Pseudomonas aeruginosa

Glutamine synthetase from Pseudomonas aeruginosa is regulated by repression/derepression of enzyme synthesis and by adenylylation/deadenylylation control. High levels of deadenylylated biosynthetically active glutamine synthetase were observed in cultures growing with limiting amounts of nitrogen while synthesis of the enzyme was repressed and that present was adenylylated in cultures with excess nitrogen. NADP- and NAD-dependent glutamate dehydrogenase could be separated by column chromatography and showed molecular weights of 110,000 and 220,000, respectively. Synthesis of the NADP-dependent glutamate dehydrogenase is repressed under nitrogen limitation and by growth on glutamate. In contrast, NAD-dependent glutamate dehydrogenase is derepressed by glutamate. Glutamate synthase is repressed by glutamate but not by excess nitrogen.

Selection of Bacillus subtilis mutants impaired in ammonia assimilation

The selection of Bacillus subtilis mutants capable of using D-histidine to fulfill a requirement for L-histidine resulted in mutants with either no glutamate synthase activity or increased amounts of an altered glutamine synthetase.

Salmonella typhimurium mutants with altered glutamate dehydrogenase and glutamate synthase activities

Although glutamate is a key compound in nitrogen metabolism, little is known about the function or regulation of its two biosynthetic enzymes, glutamate dehydrogenase and glutamate synthase. To begin the characterization of glutamate formation in Salmonella typhimurium, we isolated mutants having altered glutamate dehydrogenase and glutamate synthase activities. Mutants which failed to grow on media with glucose as the carbon source and less than 1 mM (NH(4))(2)SO(4) as the nitrogen source (Asm(-)) had about one-fourth the normal glutamate synthase activity and one-half the glutamine synthetase activity. The asm mutations also prevented growth with alanine, arginine, or proline as nitrogen sources and conferred resistance to methionine sulfoximine. When a mutation (gdh-51) causing the loss of glutamate dehydrogenase activity was transferred into a strain with an asm-102 mutation, the resulting asm-102 gdh-51 mutant had a partial requirement for glutamate. A strain isolated as a complete glutamate auxotroph had a third mutation, in addition to the asm-102 gdh-51 lesions, that further decreased the glutamate synthase activities to 1/20 the normal level. Both the asm-102 and gdh-51 mutations were located on the S. typhimurium linkage map at sites distinct from those found for mutations causing similar phenotypes in Klebsiella aerogenes and Escherichia coli.

Glutamine synthetase mutations which affect expression of nitrogen fixation genes in Klebsiella pneumoniae

Previous studies have implicated glutamine synthetase (L-glutamate:ammonia ligase [adenosine diphosphate for-ing], EC 6.6.1.2) as a major controlling element of the nitrogen fixation (nif) genes in Klebsiella pneumoniae. We report here the isolation of a new class of K. pneumoniae mutants which exhibit altered patterns of nif and hut (histidine utlization) regulation. The expression of nif in these mutants, which were isolated as Gln+ (glutamine nonrequiring) revertants of a particular glnA mutation, is extremely sensitive to ammonia repression. These mutants have a Nif- Hut- phenotype at external ammonia concentrations at which wild-type strains are Nif+ Hut+. On the other hand, these mutants can be fully derepressed for nif at very low ammonia concentrations. We adopted the nomenclature "GlnR- (Nif- Hut-)" to facilitate discussion of the phenotype of these mutant strains. The mutations in these strains which confer the GlnR- phenotype map at or near glnA, the structural gene for glutamine synthetase.

Nitrogen control of Salmonella typhimurium: co-regulation of synthesis of glutamine synthetase and amino acid transport systems

Nitrogen control in Salmonella typhimurium is not limited to glutamine synthetase but affects, in addition, transport systems for histidine, glutamine, lysine-arginine-ornithine, and glutamate-aspartate. Synthesis of both glutamine synthetase and transport proteins is elevated by limitation of nitrogen in the growth medium or as a result of nitrogen (N)-regulatory mutations. Increases in the amounts of these proteins were demonstrated by direct measurements of their activities, by immunological techniques, and by visual inspection of cell fractions after gel electrophoresis. The N-regulatory mutations are closely linked on the chromosome to the structural gene for glutamine synthetase, glnA: we discuss the possibility that they lie in a regulatory gene, glnR, which is distinct from glnA. Increases in amino acid transport in N-regulatory mutant strains were indicated by increased activity in direct transport assays, improved growth on substrates of the transport systems, and increased sensitivity to inhibitory analogs that are trnasported by these systems. Mutations to loss of function of individual transport components (hisJ, hisP, glnH, argT) were introduced into N-regulatory mutant strains to determine the roles of these components in the phenotype and transport behavior of the strains. The structural gene for the periplasmic glutamine-binding protein, glnH, was identified, as was a gene argT that probably encodes the structure of the lysine-arginine-ornithine-binding protein. Genes encoding the structures of the histidine- and glutamine-binding proteins are not linked to glnA or to each other by P22-mediated transduction; thus, nitrogen control is exerted on several unlinked genes.

Inhibition of growth by imidazol(on)e propionic acid: evidence in vivo for coordination of histidine catabolism with the catabolism of other amino acids

Imidazole propionic acid (ipa), a gratuitous inducer of the histidine-utilization (hut) system in Salmonella typhimurium, inhibits the organism's growth on succinate minimal medium. Induction of the hut system is necessary, but not sufficient, to cause inhibition. A study of the ability of single amino acids to relieve ipa-restricted growth suggests that insufficient glutamate is the cause of slow growth. The inhibition of growth by imidazolone propionic acid (iopa), an intermediate in the catabolism of histidine to glutamate, is similar to that by ipa. Studies using 2, 3, 5-triphenyl tetrazolium chloride plates to examine amino acid catabolism suggest that accumulation of ipa or iopa leads to inactivation of aspartate amino-transferase (AAT). This interpretation is supported by studies of an Escherichia coli mutant lacking AAT. The mutant grows poorly on succinate minimal medium, and the poor growth is relieved by the same amino acids that relieve ipa- and iopa-restricted growth. These and other findings are discussed in terms of coordination of the histidine-utilization system with enzymatic activities involved in the catabolism of other amino acids.

Salmonella typhimurium LT-2 mutants with altered glutamine synthetase levels and amino acid uptake activities

To determine whether Salmonella typhimurium has a nitrogen control response, we have examined the regulation of nitrogen utilization in two mutants with fivefold and threefold elevations in their glutamine synthetase activities. The mutants do not require glutamine for growth on glucose--ammonia medium but do have altered growth on other nitrogen sources. They grow better than an isogenic control on media containing arginine or asparate, but more slowly with proline or alanine as nitrogen sources. This unusual growth pattern is not due to altered regulation of the ammonia assimilatory enzymes, glutamate dehydrogenase and glutamate synthase, or to changes in the enzymes for aspartate degradation. However, transport for several amino acids may be affected. Measurement of amino acid uptake show that the mutants with high glutamine synthetase levels have increased rates for glutamine, arginine, aspartate, and lysine, but a decreased rate for proline. The relationship between glutamine synthetase levels and uptake was examined in two mutants with reduced, rather than increased, glutamine synthetase production. The uptake rates for glutamine and lysine were lower in these two glutamine auxotrophs than in the Gln+ controls. These results show a correlation between the glutamine synthetase levels and the uptake rates for several amino acids. In addition, the pleiotropic growth of the mutants with elevated glutamine synthetase activities suggests that a nitrogen control response exists for S. typhimurium and that it can be altered by mutations affecting glutamine synthetase regulation.