Regulation of histidase synthesis in intergeneric hybrids of enteric bacteria

Metabolomic analysis of riboswitch containing E. coli recombinant expression system

In this study we have employed metabolomics approaches to understand the metabolic effects of producing enhanced green fluorescent protein (eGFP) as a recombinant protein in Escherichia coli cells. This metabolic burden analysis was performed against a number of recombinant expression systems and control strains and included: (i) standard transcriptional recombinant expression control system BL21(DE3) with the expression plasmid pET-eGFP, (ii) the recently developed dual transcriptional-translational recombinant expression control strain BL21(IL3), with pET-eGFP, (iii) BL21(DE3) with an empty expression plasmid pET, (iv) BL21(IL3) with an empty expression plasmid, and (v) BL21(DE3) without an expression plasmid; all strains were cultured under various induction conditions. The growth profiles of all strains together with the results gathered by the analysis of the Fourier transform infrared (FT-IR) spectroscopy data, identified IPTG-dependent induction as the dominant factor hampering cellular growth and metabolism, which was in general agreement with the findings of GC-MS analysis of cell extracts and media samples. In addition, the exposure of host cells to the synthetic inducer ligand, pyrimido[4,5-d] pyrimidine-2,4-diamine (PPDA), of the orthogonal riboswitch containing expression system (BL21(IL3)) did not display any detrimental effects, and its detected levels in all the samples were at similar levels, emphasising the inability of the cells to metabolise PPDA. The overall results obtained in this study suggested that although the BL21(DE3)-EGFP and BL21(IL3)-EGFP strains produced comparable levels of recombinant eGFP, the presence of the orthogonal riboswitch seemed to be moderating the metabolic burden of eGFP production in the cells enabling higher biomass yield, whilst providing a greater level of control over protein expression.

Regulation of the histidine utilization (hut) system in bacteria

The ability to degrade the amino acid histidine to ammonia, glutamate, and a one-carbon compound (formate or formamide) is a property that is widely distributed among bacteria. The four or five enzymatic steps of the pathway are highly conserved, and the chemistry of the reactions displays several unusual features, including the rearrangement of a portion of the histidase polypeptide chain to yield an unusual imidazole structure at the active site and the use of a tightly bound NAD molecule as an electrophile rather than a redox-active element in urocanase. Given the importance of this amino acid, it is not surprising that the degradation of histidine is tightly regulated. The study of that regulation led to three central paradigms in bacterial regulation: catabolite repression by glucose and other carbon sources, nitrogen regulation and two-component regulators in general, and autoregulation of bacterial regulators. This review focuses on three groups of organisms for which studies are most complete: the enteric bacteria, for which the regulation is best understood; the pseudomonads, for which the chemistry is best characterized; and Bacillus subtilis, for which the regulatory mechanisms are very different from those of the Gram-negative bacteria. The Hut pathway is fundamentally a catabolic pathway that allows cells to use histidine as a source of carbon, energy, and nitrogen, but other roles for the pathway are also considered briefly here.

Purification and characterization of an extracellular protease from the fish pathogen Yersinia ruckeri and effect of culture conditions on production

A novel protease, hydrolyzing azocasein, was identified, purified, and characterized from the culture supernatant of the fish pathogen Yersinia ruckeri. Exoprotease production was detected at the end of the exponential growth phase and was temperature dependent. Activity was detected in peptone but not in Casamino Acid medium. Its synthesis appeared to be under catabolite repression and ammonium control. The protease was purified in a simple two-step procedure involving ammonium sulfate precipitation and ion-exchange chromatography. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis of the purified protein indicated an estimated molecular mass of 47 kDa. The protease had characteristics of a cold-adapted protein, i.e., it was more active in the range of 25 to 42 degrees C and had an optimum activity at 37 degrees C. The activation energy for the hydrolysis of azocasein was determined to be 15.53 kcal/mol, and the enzyme showed a rapid decrease in activity at 42 degrees C. The enzyme had an optimum pH of around 8. Characterization of the protease showed that it required certain cations such as Mg(2+) or Ca(2+) for maximal activity and was inhibited by EDTA, 1,10-phenanthroline, and EGTA but not by phenylmethylsulfonyl fluoride. Two N-methyl-N-nitro-N-nitrosoguanidine mutants were isolated and analyzed; one did not show caseinolytic activity and lacked the 47-kDa protein, while the other was hyperproteolytic and produced increased amounts of the 47-kDa protein. Azocasein activity, SDS-PAGE, immunoblotting by using polyclonal anti-47-kDa-protease serum, and zymogram analyses showed that protease activity was present in 8 of 14 strains tested and that two Y. ruckeri groups could be established based on the presence or absence of the 47-kDa protease.

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.

The nac (nitrogen assimilation control) gene from Escherichia coli

The nitrogen assimilation control gene, nac, was detected in Escherichia coli but not in Salmonella typhimurium by Southern blotting, using a probe from the Klebsiella aerogenes nac (nacK) gene. The E. coli nac gene (nacE) was isolated from a cosmid clone by complementation of a nac mutation in K. aerogenes. nacE was fully functional in this complementation assay. DNA sequence analysis showed considerable divergence between nacE and nacK, with a predicted amino acid sequence identity of only 79% and most of the divergence in the C-terminal half of the protein sequence. The total predicted size of NAC(E) is 305 amino acids, the same as for NAC(K). A null mutation, nac-28, was generated by reverse genetics. Mutants bearing nac-28 have a variety of phenotypes related to nitrogen metabolism, including slower growth on cytosine, faster growth on arginine, and suppression of the failure of an Ntr-constitutive mutant to grow with serine as sole nitrogen source. In addition to a loss of nitrogen regulation of histidase formation, nac-28 mutants also showed a loss of a weak repression of glutamate dehydrogenase formation. This repression was unexpected because it is balanced by a NAC-independent activation of glutamate dehydrogenase formation during nitrogen-limited growth. Attempts to purify NAC(E) by using methods established for NAC(K) failed, and NAC(E) appears to be degraded with a half-life at 30 degrees C as short as 15 min during inhibition of protein synthesis.

Activation of transcription initiation from the nac promoter of Klebsiella aerogenes

The nac gene of Klebsiella aerogenes encodes a bifunctional transcription factor that activates or represses the expression of several operons under conditions of nitrogen limitation. In experiments with purified components, transcription from the nac promoter was initiated by sigma 54 RNA polymerase and was activated by the phosphorylated form of nitrogen regulator I (NRI) (NtrC). The activation of the nac promoter required a higher concentration of NRI approximately P than did the activation of the Escherichia coli glnAp2 promoter, and both the promoter and upstream enhancer element contributed to this difference. The nac promoter had a lower affinity for sigma 54 RNA polymerase than did glnAp2, and uninitiated competitor-resistant transcription complexes formed at the nac promoter decayed to competitor-sensitive complexes at a greater rate than did similar complexes formed at the glnAp2 promoter. The nac enhancer, consisting of a single high-affinity NRI-binding site and an adjacent site with low affinity for NRI, was less efficient in stimulating transcription than was the glnA enhancer, which consists of two adjacent high-affinity NRI-binding sites. When these binding sites were exchanged, transcription from the nac promoter was increased and transcription from the glnAp2 promoter was decreased at low concentrations of NRI approximately P. Another indication of the difference in the efficiency of these enhancers is that although activation of a nac promoter construct containing the glnA enhancer was relatively insensitive to subtle alterations in the position of these sites relative to the position of the promoter, activation of the natural nac promoter or a nac promoter construct containing only a single high-affinity NRI approximately P binding site was strongly affected by subtle alterations in the position of the NRI approximately P binding site(s), indicating a face-of-the-helix dependency for activation.

The role of the NAC protein in the nitrogen regulation of Klebsiella aerogenes

The NAC (nitrogen assimilation control) protein from Klebsiella aerogenes is a LysR-like regulator for transcription of several operons involved in nitrogen metabolism, and couples the transcription of these sigma 70-dependent operons to regulation by the sigma 54-dependent NTR system. NAC activates expression of operons (e.g. histidine utilization, hut), allowing use of poor nitrogen sources, and represses expression of operons (e.g. glutamate dehydrogenase, gdh) allowing assimilation of the preferred nitrogen source, ammonium. NAC is both necessary and sufficient to activate transcription, but the expression of the nac gene is totally dependent on the central nitrogen regulatory system (NTR) and RNA polymerase carrying the sigma 54 sigma factor (RNAP sigma 54). Nitrogen starvation signals the NTR system to transcribe nac, and NAC activates the transcription of hut, put (proline utilization), and urease. NAC does not affect the transcription of RNAP sigma 54-dependent operons like ginA or nifLA, which respond directly to the NTR system, but activates transcription of RNAP sigma 70-dependent operons. Thus NAC acts as a bridge between RNAP sigma 70-dependent operons like hut and the RNAP sigma 54-dependent NTR system. The activation of operons like hut by NAC in response to nitrogen starvation is at least superficially similar to their activation by CAP-cAMP in response to carbon and energy starvation.

Klebsiella aerogenes catabolite gene activator protein and the gene encoding it (crp)

The catabolite gene activator protein from Klebsiella aerogenes (CAPK) and the corresponding protein from Escherichia coli (CAPE) were shown to be nearly identical. Both CAPK and CAPE activated transcription from the CAP-dependent promoters derived from E. coli and K. aerogenes. The crp gene from K. aerogenes (encoding CAP) is tightly linked to rpsL. The nucleotide sequence of crp predicts an amino acid sequence for CAPK that differs in only one position from that of CAPE.

Cloning of the Klebsiella aerogenes nac gene, which encodes a factor required for nitrogen regulation of the histidine utilization (hut) operons in Salmonella typhimurium

The nac (nitrogen assimilation control) gene from Klebsiella aerogenes, cloned in a low-copy-number cloning vector, restored the ability of K. aerogenes nac mutants to activate histidase and repress glutamate dehydrogenase formation in response to nitrogen limitation and to limit the maximum expression of the nac promoter. When present in Salmonella typhimurium, the K. aerogenes nac gene allowed the hut genes to be activated during nitrogen-limited growth. Thus, the nac gene encodes a cytoplasmic factor required for activation of hut expression in S. typhimurium during nitrogen-limited growth.

Tn1000-mediated insertion mutagenesis of the histidine utilization (hut) gene cluster from Klebsiella aerogenes: genetic analysis of hut and unusual target specificity of Tn1000

The histidine utilization (hut) genes from Klebsiella aerogenes were cloned in both orientations into the HindIII site of plasmid pBR325, and the two resulting plasmids, pCB120 and pCB121, were subjected to mutagenesis with Tn1000. The insertion sites of Tn1000 into pCB121 were evenly distributed throughout the plasmid, but the insertion sites into pCB120 were not. There was a large excess of Tn1000 insertions in the "plus" or gamma delta orientation in a small, ca. 3.5-kilobase region of the plasmid. Genetic analysis of the Tn1000 insertions in pCB120 and pCB121 showed that the hutUH genes form an operon transcribed from hutU and that the hutC gene (encoding the hut-specific repressor) is independently transcribed from its own promoter. The hutIG cluster appears not to form an operon. Curiously, insertions in hutI gave two different phenotypes in complementation tests against hutG504, suggesting either that hutI contains two functionally distinct domains or that there may be another undefined locus within the hut cluster. The set of Tn1000 insertions allowed an assignment of the gene boundaries within the hut cluster, and minicell analysis of the polypeptides expressed from plasmids carrying insertions in the hut genes showed that the hutI, hutG, hutU, and hutH genes encode polypeptides of 43, 33, 57, and 54 kilodaltons, respectively.

Regulation of the galactose-inducible lac operon and the histidine utilization operons in pts mutants of Klebsiella aerogenes

Galactose appears to be the physiological inducer of the chromosomal lac operon in Klebsiella aerogenes. Both lactose and galactose are poor inducers in strains having a functional galactose catabolism (gal) operon, but both are excellent inducers in gal mutants. Thus the slow growth of K. aerogenes on lactose reflects the rapid degradation of the inducer. Several pts mutations were characterized and shown to affect both inducer exclusion and permanent catabolite repression. The beta-galactosidase of pts mutants cannot be induced at all by lactose, and pts mutants appear to have a permanent and constitutive inducer exclusion phenotype. In addition, pts mutants show a reduced rate of glucose metabolism, leading to slower growth on glucose and a reduced degree of glucose-mediated permanent catabolite repression. The crr-type pseudorevertants of pts mutations relieve the constitutive inducer exclusion for lac but do not restore the full level of glucose-mediated permanent catabolite repression and only slightly weaken the glucose-mediated inducer exclusion. Except for weakening the glucose-mediated permanent catabolite repression, pts and crr mutations have no effect on expression of the histidine utilization (hut) operons.

Organization and multiple regulation of histidine utilization genes in Pseudomonas putida

The arrangement of the histidine utilization (hut) genes in Pseudomonas putida was established by examining the structure of a DNA segment that had been cloned into Escherichia coli via a cosmid vector. Southern blot analysis revealed that the restriction patterns of the hut genes cloned into E. coli and present in the P. putida genome were identical, indicating that no detectable DNA rearrangement took place during the cloning. Expression of the hut genes from a series of overlapping clones indicated the gene order to be hutG-hutI-hutH-hutU-hutC-hutF. The transcription directions of the different hut genes were determined by cloning the genes under control of the lambda pL promoter. This showed that hutF, encoding formiminoglutamate hydrolase, was transcribed in a direction opposite to that of the other genes. Inactivation of the cloned hut genes by Tn1000 insertion revealed that the hut genes were divided into three major transcriptional units (hutF, hutC [the repressor gene], and hut UHIG), but hutG may also be independently transcribed. When cloned individually with hutC on the same vector, hutF and hutU (which encodes urocanase) expression was induced by urocanate, indicating that these two genes each possess an operator-promoter element. Tn1000 insertions (in the cloned genes) or Tn5 insertions (in the P. putida genome) affecting the hutI or hutH gene only partially eliminated hutG expression. Furthermore, hutG, which specifies N-formylglutamate amidohydrolase, was regulated by the hutC product when the two genes were cloned on the same vector and expressed in E. coli. Therefore, hutG can be expressed independently from its own promoter, in keeping with earlier observations that N-formylglutamate amidohydrolase synthesis is not coordinated with that of urocanase and histidase and can be induced by N-formylglutamate or urocanate.

Cloning and expression in Escherichia coli of histidine utilization genes from Pseudomonas putida

A library of the Pseudomonas putida chromosome, prepared through the use of the cosmid pJB8 ligated to a partial Sau3A digest of bacterial DNA, followed by in vitro packaging into bacteriophage lambda particles, was used to construct a strain of Escherichia coli which contained the genes for histidine utilization. This isolate produced a repressor product and all five enzymes required in Pseudomonas spp. for histidine dissimilation, whereas none of these could be detected in the nontransduced parent E. coli strain. When this transductant was grown on various media containing histidine or urocanate as the inducer, it was observed that production of the cloned histidine degradative enzymes was influenced somewhat by the choice of nitrogen source used but not by the carbon source. The recombinant cosmid was isolated and found to consist of 21.1 kilobase pairs of DNA, with approximately 16 kilobase pairs derived from Pseudomonas DNA and the remainder being from the pJB8 vector. Digestion of this insert DNA with EcoRI provided a 6.1-kilobase-pair fragment which, upon ligation in pUC8 and transformation into an E. coli host, was found to encode histidine ammonia-lyase and urocanase. The inducible nature of this production indicated that the hut repressor gene also was present on this fragment. Insertional inactivation of the histidine ammonia-lyase and urocanase genes by the gamma-delta transposon has permitted location of these structural genes and has provided evidence that transcription proceeds from urocanase through histidine ammonia-lyase. Mapping of the 16-kilobase-pair Pseudomonas DNA segment with restriction enzymes and subcloning of additional portions, one of which contained the gene for formiminoglutamate hydrolase and another that could constitutively express activities for both imidazolone propionate hydrolase and formylglutamate hydrolase, has provided evidence for the organization of all hut genes.

Regulation of hutUH operon expression by the catabolite gene activator protein-cyclic AMP complex in Klebsiella aerogenes

RNA polymerase transcribed the hutUH operon of Klebsiella aerogenes if the catabolite gene activator protein (CAP) and cyclic AMP (cAMP) were present or if the DNA template was derived from a promoter mutant in which hutUH expression was independent of the need for positive effectors. In the absence of CAP or cAMP, not only was hutUH transcription absent, but transcription in the opposite direction (toward hutC) was initiated at a site (pC) ca. 70 base pairs from the site (pUH) of hutUH mRNA initiation. When the pC promoter was cloned in front of a promoterless galK gene, active expression of galK was observed. Thus, the pC promoter is active in vivo as well as in vitro. Transcription from pUH and pC may be mutually exclusive, with the major effect of CAP and cAMP being to prevent transcription from pC, thus relieving the antagonistic effect on transcription from pUH. This "double-negative" control by CAP-cAMP is supported by two observations: (i) CAP-cAMP was unable to activate transcription from pUH if RNA polymerase had been previously bound to pC and (ii) a mutation that allowed transcription from pUH in the absence of positive effectors simultaneously eliminated the activity of pC. An alternative model, in which CAP-cAMP is required for pUH expression and RNA polymerase binding at pC serves to modulate this control in some unknown way, is also considered. The physiological role of the transcript from pC other than regulation of pUH is unknown.

A restriction enzyme cleavage map of the histidine utilization (hut) genes of Klebsiella aerogenes and deletions lacking regions of hut DNA

The histidine utilization (hut) operons of Klebsiella aerogenes were cloned into pBR322. The hut genes are wholly contained on a 7.9 kilobase pair fragment bounded by HindIII restriction sites and expression of hut is independent of the orientation of the fragment with respect to pBR322. A restriction map locating the 27 cleavage sites within hut for the enzymes, HindIII, PvuII, SalI, BglII, KpnI, PstI, SmaI, AvaI, and BamHI was deduced. Several of the cleavage sites for the enzymes HaeIII and HinfI were also mapped. A set of deletion plasmids was isolated by removing various restriction fragments from the original plasmid. These deletions were characterized and were used to assist in mapping restriction sites. This physical characterization of hut DNA opens the way for genetic and molecular analysis of the regulation of hut gene expression in vitro as well as in vivo.

Messenger RNA for glutamine synthetase. Review article

Of the various eucaryotic tissues, where glutamine synthetase (GS) mRNA and its regulation have been investigated, the induction of GS by glucocorticoids in the embryonic chick retina represents one of the systems most extensively studied. GS mRNA was first identified at the polysomal level by immunochemical precipitation of fractionated polysomes containing nascent GS chains with anti-GS gamma-globulin. The mRNA has been shown to be polyadenylated at the 3' end; on this basis, it has been partially purified from embryonic chick retina as well as from N. Crassa by chromatography on oligo(dT)-cellulose or poly(U)-sepharose and translated in cell-free protein synthesizing systems derived from wheat germ. Hormonal regulation of GS activity studied in the embryonic retina, hepatoma tissue culture cells, or in other tissues is always shown to be mediated by GS mRNA. In the retina, hydrocortisone (HC) elicits an age-related and transcription-dependent induction of GS by enhancing the level of GS mRNA in the polysomes through an increased supply of this mRNA from the nucleus. Comparative studies of three inhibitors of transcription, viz. actinomycin D, leucanthone and proflavine on the induction of GS by HC indicate that the latter inhibits GS mRNA selectively and reversibly with minimal effects on other RNA synthesis. Since proflavine acts by competing with HC-receptor binding sites in the nuclei, further studies on its interaction with the retina genome are likely to help identify the DNA sequences involved in the GS induction. In bacteria, studies on the genetics and physiology of various mutants with lesions in the structural gene for GS show that the transcription of the GS gene (gln A) is regulated both positively and negatively by GS and the product of another gene gln F. Purification of GS mRNA to homogeneity cloning of its cDNA and development of assay systems for cell-free transcription of GS are other studies likely to advance our knowledge on GS mRNA and its regulation.

Regulation of nitrogen catabolic enzymes in Bacillus spp

The levels of the inducible nitrogen catabolic enzymes arginase (L-arginine amidinohydrolase, EC 3.5.3.1) and alanine dehydrogenase (L-alanine:NAD+ oxidoreductase [deaminating], EC 1.4.1.1) from Bacillus licheniformis and histidase (L-histidine ammonia-lyase, EC 4.3.1.3) from Bacillus subtilis and the ammonia assimilatory enzymes from B. licheniformis were determined in cultures grown in the presence of different nitrogen sources. Although the levels of these enzymes were dependent upon the nitrogen source present, induction of the catabolic enzymes in response to the addition of inducer occurred even in the presence of preferred nitrogen sources. Intracellular pool sizes of ammonia, glutamate, glutamine, and alpha-ketoglutarate were measured in continuous cultures of b. licheniformis growing in the presence of different nitrogen sources. A comparison of the pool sizes of these metabolites with the ammonia assimilatory enzyme levels showed that the pools of the metabolites did not change in a manner consistent with their use as regulators of the synthesis of any of these enzymes.

Regulation of derepressed synthesis of arylsulfatase by tyramine oxidase in Salmonella typhimurium

The participation of tyramine oxidase in the regulation of arylsulfatase synthesis in Salmonella typhimurium was studied. Arylsulfatase synthesis was repressed by inorganic sulfate, cysteine, methionine, or taurine. This repression was relieved by tyramine, octopamine, or dopamine, which induced tyramine oxidase synthesis, although the level of arylsulfatase activity was very low. The induction of tyramine oxidase and derepression of arylsulfatase by tyramine were strongly inhibited by glucose and ammonium chloride, and the repression of both enzymes was relieved by use of xylose as a carbon source after consumption of glucose or by use of tyramine as the sole source of nitrogen, irrespective of the carbon source used. The initial rates of tyramine uptake by cells grown with glucose and xylose were similar. Results with tyramine oxidase-constitutive mutants showed that constitutive expression of the tyramine oxidase gene resulted in derepression of arylsulfatase synthesis in the absence of tyramine. Thus, catabolite and ammonium repressions of arylsulfatase synthesis and the induction of the enzyme by tyramine seem to reflect the levels of tyramine oxidase synthesis. These results in S. typhimurium support our previous finding that the specific regulation system of arylsulfatase synthesis by tyramine oxidase is conserved in enteric bacteria.

Tryptophan metabolism in Klebsiella aerogenes: regulation of the utilization of aromatic amino acids as sources of nitrogen

Klebsiella aerogenes utilized aromatic amino acids as sole sources of nitrogen but not as sole sources of carbon. K. aerogenes abstracted the alpha-amino group of these compounds by transamination and excreted the arylpyruvate portions into the medium. When tryptophan was utilized as the sole source of nitrogen by K. aerogenes, indolepyruvate was excreted into the medium, where it polymerized non-enzymatically to form a brick red pigment. At least four separate aromatic aminotransferase activities were found in K. aerogenes. One activity (aromatic aminotransferase I) appeared to be solely responsible for the aminotransferase reaction necessary for the growth of K. aerogenes when tryptophan was the source of nitrogen; the loss of this activity by mutation (tut) prevented the growth of cells on media containing this and other aromatic amino acids. None of the other aminotransferase activities in the cells could substitute for aromatic aminotransferase in this regard. Tryptophan-dependent pigment formation in K. aerogenes was positively controlled by the intracellular level of glutamine synthetase. Nevertheless, the aromatic aminotransferase activity in cells varied less than 2-fold in response to 10-fold or greater changes in the levels of glutamine synthetase. Glutamine synthetase affected the ability of the cells to take up tryptophan from the medium.

Regulation of expression from the glnA promoter of Escherichia coli in the absence of glutamine synthetase

One of the suspected regulators of glutamine synthetase [L-glutamate:ammonia ligase (ADP-forming), EC 6.3.1.2] in enteric bacteria is glutamine synthetase itself. We isolated Escherichia coli strains carrying fusions of the beta-galactosidase structural gene to the promoter of the glutamine synthetase gene, with the aid of the Casadaban Mud1 (ApR, lac, cts62) phage. Some aspects of regulation were retained in haploid fusion strains despite the absence of glutamine synthetase, whereas other aspects required glutamine synthetase catalytic or regulatory activity or both. The direction of transcription of the glutamine synthetase gene was also determined.

Regulation of nitrogen metabolism in glutamine auxotrophs of Klebsiella pneumoniae

We examined the regulation of nitrogen metabolism in four classes (glnA, glnB, glnF, and glnG) of Gln- auxotrophs of Klebsiella pneumoniae. These studies indicate that glutamine synthetase does not directly mediate the physiological response to NH4+ in this organism. We present evidence suggesting that the effect of NH4+ on the expression of genes involved in nitrogen metabolism involves the products of the glnF and glnG genes.

Regulation of Klebsiella pneumoniae hut operons by oxygen

We investigated the regulation of genes concerned with nitrogen metabolism by oxygen in the facultative anaerobe Klebsiella pneumoniae. We found oxygen to be required for the expression of the hut operons; the effect of O2 on the glutamine synthetase and urease was less pronounced than on the hut operons. Glutamine synthetase was transiently repressed during the transition from an aerobic to an anaerobic environment. Regulation of hut by O2 suppressed the effect of nitrogen limitation on the expression of these genes.

Relation between the adenylylation state of glutamine synthetase and the expression of other genes involved in nitrogen metabolism

We have partially characterized the biochemical parameters of glutamine synthetase from Klebsiella pneumoniae and have shown that the differential affinity of adenylylated and unadenylylated glutamine synthetase for adenosine diphosphate provides a convenient means of determining the adenylylation state. Using this assay procedure, we examined the relationship between the adenylylation state and the expression of other genes involved in nitrogen assimilation. We observed no correlation between the adenylylation state and the expression of histidase, glutamine synthetase, glutamate synthase, glutamate dehydrogenase, and urease in aerobic cultures.

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.

Regulation of gamma-aminobutyric acid degradation in Escherichia coli by nitrogen metabolism enzymes

The possible role of glutamate dehydrogenase, glutamate synthase, and glutamine synthetase in the regulation of enzyme formation in the gamma-aminobutyric acid (GABA) catabolic pathway of Escherichia coli K-12 was investigated. Evidence is presented indicating that glutamine synthetase acts as a positive regulator in the E. coli GABA control system. Mutations impairing glutamate synthase activity prevent the depression of the enzymes of the GABA pathway in ammonia-limited glucose media. However, mutations resulting in constitutive synthesis of glutamine synthetase (GlnC) restore the ability of the glutamate synthase-less mutants to grow in glucose-GABA media and result in depressed synthesis of the GABA enzymes. It is suggested that the loss of glutamate synthesis activity affects the GABA control system indirectly by lowering glutamine synthetase levels.

gltB gene and regulation of nitrogen metabolism by glutamine synthetase in Escherichia coli

A mutant (gltB) of Escherichia coli lacking glutamate synthase (GOGAT) was unable to utilize a wide variety of compounds as sole nitrogen source (e.g., arginine, proline, gamma-aminobutyrate, and glycine). Among revertants of these Asm- strains selected on one of these compounds (e.g., arginine, proline, or gamma-aminobutyrate) were those that produce glutamine synthetase (GS) constitutively (GlnC phenotype). These revertants had a pleiotropically restored ability to grow on compounds that are metabolized to glutamate. This suggested that the expression of the genes responsible for the metabolism of these nitrogen sources was regulated by GS. An examination of the regulation of proline oxidase confirmed this hypothesis. The differential sensitivities of GlnC and wild-type strains to low concentrations (0.1 mM) of the glutamine analog L-methionine-DL-sulfoximine supported the conclusion that the synthesis of a glutamine permease was also positively controlled by GS. During the course of this study we found that the reported position of the locus (gltB) for glutamate synthase is incorrect. We have relocated this gene to be 44% linked to the argG locus by P1 transduction. Further mapping has shown that the locus previously called aspB is in reality the gltB locus and that the "suppressor" of the aspB mutation (A. M. Reiner, J. Bacteriol. 97:1431-1436, 1969) is the locus for glutamate dehydrogenase (gdhA).

Regulation of enzyme synthesis by the glutamine synthetase of Salmonella typhimurium: a factor in addition to glutamine synthetase is required for activation of enzyme formation

In Klebsiella aerogenes but not in Salmonella typhimurium glutamine synthetase can function during nitrogen-limited growth to increase the rate of synthesis of histidase from the hut genes of S. typhimurium 15-59 (hutS. 15-59). Formation of proline oxidase is also not increased in nitrogen-limited cultures of S. typhimurium. However, in hybrid strains of Escherichia coli or K. aerogenes, the glutamine synthetase of S. typhimurium activates synthesis of histidase from the hutS. 15-59 genes. Apparently, glutamine synthetase is necessary but not sufficient for activation of transcription of the hut genes; another factor must also be present. This factor is active in both K. aerogenes and E. coli but is missing or altered in S. typhimurium.

Regulation of glnA messinger ribonucleic acid synthesis in Klebsiella aerogenes

We examined wild-type and mutant strains of Klebsiella aerogenes for the relative amounts of ribonucleic acid (RNA) hybridizing specifically to deoxyribonucleic acid from a transducing phage carrying glnAK, the structural gene for glutamine synthetase. Our data showed a positive correlation between the intracellular level of glutamine synthetase and the level of glnA messenger RNA; we were unable to detect glnA messinger RNA in strains devoid of glutamine synthetase protein. Therefore, it is possible that transcription of glnA is not regulated simply by repression mediated through the glutamine synthetase protein; rather, autogenous control in this system may involve activation of transcription. Our experiments also suggest that the promotor of the glnA gene is located at the rha proximal end of the gene.