Salmonella typhimurium mutants with altered glutamate dehydrogenase and glutamate synthase activities

Osmoregulation in Rhizobium meliloti: Production of Glutamic Acid in Response to Osmotic Stress

Rhizobium meliloti, like many other bacteria, accumulates high levels of glutamic acid when osmotically stressed. The effect was found to be proportional to the osmolarity of the growth medium. NaCl, KCI, sucrose, and polyethylene glycol elicited this response. The intracellular levels of glutamate and K began to increase immediately when cells were shifted to high-osmolarity medium. Antibiotics that inhibit protein synthesis did not affect this increase in glutamate production. Cells growing in conventional media at any stage in the growth cycle could be suspended in medium causing osmotic stress and excess glutamate accumulated. The excess glutamate did not appear to be excreted, and the intracellular level eventually returned to normal when osmotically stressed cells were suspended in low-osmolarity medium. A glt mutant lacking glutamate synthase and auxotrophic for glutamate accumulated excess glutamate in response to osmotic stress. Addition of isoleucine, glutamine, proline, or arginine stimulated glutamate accumulation to wild-type levels when the mutant cells were suspended in minimal medium with NaCl to cause osmotic stress. In both wild-type and mutant cells, inhibitors of transaminase activity, including azaserine and aminooxyacetate, reduced glutamate levels. The results suggest that the excess glutamate made in response to osmotic stress is derived from degradation of amino acids and transamination of 2-ketoglutarate.

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.

Helicobacter pylori physiology predicted from genomic comparison of two strains

Helicobacter pylori is a gram-negative bacteria which colonizes the gastric mucosa of humans and is implicated in a wide range of gastroduodenal diseases. This paper reviews the physiology of this bacterium as predicted from the sequenced genomes of two unrelated strains and reconciles these predictions with the literature. In general, the predicted capabilities are in good agreement with reported experimental observations. H. pylori is limited in carbohydrate utilization and will use amino acids, for which it has transporter systems, as sources of carbon. Energy can be generated by fermentation, and the bacterium possesses components necessary for both aerobic and anaerobic respiration. Sulfur metabolism is limited, whereas nitrogen metabolism is extensive. There is active uptake of DNA via transformation and ample restriction-modification activities. The cell contains numerous outer membrane proteins, some of which are porins or involved in iron uptake. Some of these outer membrane proteins and the lipopolysaccharide may be regulated by a slipped-strand repair mechanism which probably results in phase variation and plays a role in colonization. In contrast to a commonly held belief that H. pylori is a very diverse species, few differences were predicted in the physiology of these two unrelated strains, indicating that host and environmental factors probably play a significant role in the outcome of H. pylori-related disease.

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)

Improvement of Nitrogen Supply for L-Threonine Production by a Recombinant Strain of Serratia marcescens

Serratia marcescens T-2000 was previously reported to be an L-threonine-producing strain that harbors the recombinant plasmid carrying the mutant-type threonine operon. This strain produced 55 g of L-threonine/L of the medium containing urea as a nitrogen source after 72 h of cultivation. In the urea-containing medium, transitory stop of the growth was observed during the early period of cultivation when the entire amount of ammonium ion formed from urea via heat decomposition disappeared in the medium. This indicated that the shortage of ammonium supply in cells might delay both the cell growth and the L-threonine production. The use of ammonia water as a nitrogen source for L-threonine production was therefore studied, because microbial cells generally assimilate this source more readily than urea. When ammonia water was automatically fed to the medium so as to maintain the pH of the medium at around 7, the growth was accelerated, and the L-threonine production reached a maximum of 65 g/L at 48 h. Under these conditions, sucrose, a carbon source, was continuously fed to the medium, resulting in the production of 100 g of L-threonine/L at 96 h. Thus, the L-threonine production of the recombinant L-threonine-producing strain could be increased by devising the method for supply of a nitrogen source.

Organic nitrogen metabolism of phototrophic bacteria

Recent reviews dealing with phototrophic bacteria are concerned with bioenergetics, nitrogen fixation and hydrogen metabolism, synthesis of the photosynthetic apparatus and phylogeny/taxonomy. The organic N-metabolism of these phylogenetically diverse bacteria has last been reviewed in 1978. However, amino acid utilization and biosynthesis, ammonia assimilation, purine and pyrimidine metabolism and biosynthesis of delta-aminolevulinic acid as precursor of bacteriochlorophylls and hemes are topics of vital importance. This review focuses on utilization of amino acids as N- and C/N-sources, the pathways of purine and pyrimidine degradation, novel aspects of amino acid biosynthesis (with emphasis on branched-chain amino acids and delta-aminolevulinic acid) and some aspects of ammonia assimilation and glutamate synthesis by purple bacteria, green sulfur bacteria and Chloroflexus aurantiacus.

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.

Proline-hyperproducing strains of Serratia marcescens: enhancement of proline analog-mediated growth inhibition by increasing osmotic stress

Proline-producing strains of Serratia marcescens were more osmotolerant than wild-type strains. Growth inhibition by proline analogs was significantly enhanced by increasing the osmotic stress of the medium. Mutants resistant to azetidine-2-carboxylate were derived from a proline-producing strain, SP126, under a high osmotic condition. One of the mutants, strain SP187, produced 56 mg of L-proline per ml of medium containing sucrose and urea. This amount was ca. 3 times larger than that produced by strain SP126. The intracellular glutamate content which decreased in strain SP126 was restored in strain SP187. The glutamate dehydrogenase level of strain SP187 was 5 times higher than that of strain SP126.

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.

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.

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.

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.

Bacteriophage P1 as a vehicle for Mu mutagenesis of Salmonella typhimurium

We developed a procedure using bacteriophage P1 as a vector for transferring Mu phage deoxyribonucleic acid into Salmonella typhimurium. Mu phage transferred in this manner yielded lysogenic auxotrophs, and we demonstrated that specific deletions and lac gene fusions can be selected.

Regulation of nitrogen utilization of hisT mutants of Salmonella typhimurium

Mutations in the hisT gene of Salmonella typhimurium alter pseudouridine synthetase I, the enzyme that modifies two uridines in the anticodon loop of numerous transfer ribonucleic acid species. We have examined two strains carrying different hisT mutations for their ability to grow on a variety of nitrogen sources. The hisT mutants grew more rapidly than did hisT+ strains with either arginine or proline as the nitrogen source and glucose as the carbon source. The hisT mutations were transduced into new strains to show that these growth properties were due to the hisT mutations. The hisT mutations did not influence the growth of mutants having altered glutamine synthetase regulation. Assays of the three primary ammonia-assimilatory enzymes, glutamate dehydrogenase, glutamine synthetase, and glutamate synthase, showed that glutamate synthase activities were lower in hisT mutants than in isogenic hisT+ controls; however, the glutamate dehydrogenase activity was about threefold higher in the hisT strains grown in glucose-arginine medium. The results suggest that the controls for enzyme synthesis for nitrogen utilization respond either directly or indirectly to transfer ribonucleic acid species affected by the hisT mutation.