Glutamine synthetase of Klebsiella aerogenes: properties of glnD mutants lacking uridylyltransferase

Nitrogen assimilation in Escherichia coli: putting molecular data into a systems perspective

We present a comprehensive overview of the hierarchical network of intracellular processes revolving around central nitrogen metabolism in Escherichia coli. The hierarchy intertwines transport, metabolism, signaling leading to posttranslational modification, and transcription. The protein components of the network include an ammonium transporter (AmtB), a glutamine transporter (GlnHPQ), two ammonium assimilation pathways (glutamine synthetase [GS]-glutamate synthase [glutamine 2-oxoglutarate amidotransferase {GOGAT}] and glutamate dehydrogenase [GDH]), the two bifunctional enzymes adenylyl transferase/adenylyl-removing enzyme (ATase) and uridylyl transferase/uridylyl-removing enzyme (UTase), the two trimeric signal transduction proteins (GlnB and GlnK), the two-component regulatory system composed of the histidine protein kinase nitrogen regulator II (NRII) and the response nitrogen regulator I (NRI), three global transcriptional regulators called nitrogen assimilation control (Nac) protein, leucine-responsive regulatory protein (Lrp), and cyclic AMP (cAMP) receptor protein (Crp), the glutaminases, and the nitrogen-phosphotransferase system. First, the structural and molecular knowledge on these proteins is reviewed. Thereafter, the activities of the components as they engage together in transport, metabolism, signal transduction, and transcription and their regulation are discussed. Next, old and new molecular data and physiological data are put into a common perspective on integral cellular functioning, especially with the aim of resolving counterintuitive or paradoxical processes featured in nitrogen assimilation. Finally, we articulate what still remains to be discovered and what general lessons can be learned from the vast amounts of data that are available now.

The poor growth of Rhodospirillum rubrum mutants lacking PII proteins is due to an excess of glutamine synthetase activity

The P(II) family of proteins is found in all three domains of life and serves as a central regulator of the function of proteins involved in nitrogen metabolism, reflecting the nitrogen and carbon balance in the cell. The genetic elimination of the genes encoding these proteins typically leads to severe growth problems, but the basis of this effect has been unknown except with Escherichia coli. We have analysed a number of the suppressor mutations that correct such growth problems in Rhodospirillum rubrum mutants lacking P(II) proteins. These suppressors map to nifR3, ntrB, ntrC, amtB(1) and the glnA region and all have the common property of decreasing total activity of glutamine synthetase (GS). We also show that GS activity is very high in the poorly growing parental strains lacking P(II) proteins. Consistent with this, overexpression of GS in glnE mutants (lacking adenylyltransferase activity) also causes poor growth. All of these results strongly imply that elevated GS activity is the causative basis for the poor growth seen in R. rubrum mutants lacking P(II) and presumably in mutants of some other organisms with similar genotypes. The result underscores the importance of proper regulation of GS activity for cell growth.

GlnD is essential for NifA activation, NtrB/NtrC-regulated gene expression, and posttranslational regulation of nitrogenase activity in the photosynthetic, nitrogen-fixing bacterium Rhodospirillum rubrum

GlnD is a bifunctional uridylyltransferase/uridylyl-removing enzyme and is thought to be the primary sensor of nitrogen status in the cell. It plays an important role in nitrogen assimilation and metabolism by reversibly regulating the modification of P(II) proteins, which in turn regulate a variety of other proteins. We report here the characterization of glnD mutants from the photosynthetic, nitrogen-fixing bacterium Rhodospirillum rubrum and the analysis of the roles of GlnD in the regulation of nitrogen fixation. Unlike glnD mutations in Azotobacter vinelandii and some other bacteria, glnD deletion mutations are not lethal in R. rubrum. Such mutants grew well in minimal medium with glutamate as the sole nitrogen source, although they grew slowly with ammonium as the sole nitrogen source (MN medium) and were unable to fix N(2). The slow growth in MN medium is apparently due to low glutamine synthetase activity, because a DeltaglnD strain with an altered glutamine synthetase that cannot be adenylylated can grow well in MN medium. Various mutation and complementation studies were used to show that the critical uridylyltransferase activity of GlnD is localized to the N-terminal region. Mutants with intermediate levels of uridylyltransferase activity are differentially defective in nif gene expression, the posttranslational regulation of nitrogenase, and NtrB/NtrC function, indicating the complexity of the physiological role of GlnD. These results have implications for the interpretation of results obtained with GlnD in many other organisms.

The GlnD and GlnK homologues of Streptomyces coelicolor A3(2) are functionally dissimilar to their nitrogen regulatory system counterparts from enteric bacteria

Glutamine synthetase I (GSI) enzyme activity in Streptomyces coelicolor is controlled post-translationally by the adenylyltransferase (GlnE) as in enteric bacteria. Although other homologues of the Escherichia coli Ntr system (glnK, coding for a PII family protein; and glnD, coding for an uridylyltransferase) are found in the S. coelicolor genome, the regulation of the GSI activity was found to be different. The functions of glnK and glnD were analysed by specific mutants. Surprisingly, biochemical assay and two-dimensional PAGE analysis showed that modification of GSI by GlnE occurs normally in all mutant strains, and neither GlnK nor GlnD are required for the regulation of GlnE in response to nitrogen stimuli. Analysis of the post-translational regulation of GlnK in vivo by two-dimensional PAGE and mass spectrometry indicated that it is subject to both a reversible and a non-reversible modification in a direct response to nitrogen availability. The irreversible modification was identified as removal of the first three N-terminal amino acid residues of the protein, and the reversible modification as adenylylation of the conserved tyro-sine 51 residue that is known to be uridylylated in E. coli. The glnD insertion mutant expressing only the N-terminal half of GlnD was capable of adenylylating GlnK, but was unable to perform the reverse deadenylylation reaction in response to excess ammonium. The glnD null mutant completely lacked the ability to adenylylate GlnK. This work provides the first example of a PII protein that is modified by adenylylation, and demonstrates that this reaction is performed by a homologue of GlnD, previously described only as a uridylyltransferase enzyme.

Lethality of glnD null mutations in Azotobacter vinelandii is suppressible by prevention of glutamine synthetase adenylylation

GlnD is a pivotal protein in sensing intracellular levels of fixed nitrogen and has been best studied in enteric bacteria, where it reversibly uridylylates two related proteins, PII and GlnK. The uridylylation state of these proteins determines the activities of glutamine synthetase (GS) and NtrC. Results presented here demonstrate that glnD is an essential gene in Azotobacter vinelandii. Null glnD mutations were introduced into the A. vinelandii genome, but none could be stably maintained unless a second mutation was present that resulted in unregulated activity of GS. One mutation, gln-71, occurred spontaneously to give strain MV71, which failed to uridylylate the GlnK protein. The second, created by design, was glnAY407F (MV75), altering the adenylylation site of GS. The gln-71 mutation is probably located in glnE, encoding adenylyltransferase, because introducing the Escherichia coli glnE gene into MV72, a glnD(+) derivative of MV71, restored the regulation of GS activity. GlnK-UMP is therefore apparently required for GS to be sufficiently deadenylylated in A. vinelandii for growth to occur. The DeltaglnD GS(c) isolates were Nif(-), which could be corrected by introducing a nifL mutation, confirming a role for GlnD in mediating nif gene regulation via some aspect of the NifL/NifA interaction. MV71 was unexpectedly NtrC(+), suggesting that A. vinelandii NtrC activity might be regulated differently than in enteric organisms.

P(II) signal transduction proteins, pivotal players in microbial nitrogen control

The P(II) family of signal transduction proteins are among the most widely distributed signal proteins in the bacterial world. First identified in 1969 as a component of the glutamine synthetase regulatory apparatus, P(II) proteins have since been recognized as playing a pivotal role in control of prokaryotic nitrogen metabolism. More recently, members of the family have been found in higher plants, where they also potentially play a role in nitrogen control. The P(II) proteins can function in the regulation of both gene transcription, by modulating the activity of regulatory proteins, and the catalytic activity of enzymes involved in nitrogen metabolism. There is also emerging evidence that they may regulate the activity of proteins required for transport of nitrogen compounds into the cell. In this review we discuss the history of the P(II) proteins, their structures and biochemistry, and their distribution and functions in prokaryotes. We survey data emerging from bacterial genome sequences and consider other likely or potential targets for control by P(II) proteins.

Mutants of Rhizobium tropici strain CIAT899 that do not induce chlorosis in plants

Type B strains of Rhizobium tropici induce severe foliar chlorosis when applied at planting to seeds of symbiotic host and non-host dicotyledonous plants. A Tn5-induced mutant, designated CT4812, or R. tropici strain CIAT899 that was unable to induce chlorosis was isolated. Cloning and sequencing of the DNA flanking the transposon in CT4812 revealed that the Tn5 insertion is located in a gene similar to glnD, which encodes uridylyltransferase/uridylyl-removing enzyme in enteric bacteria. Two marker-exchange mutants with insertions in glnD also failed to induce chlorosis in bean (Phaseolus vulgaris) plants. The 5'-most insertion in glnD (in mutant strain ME330) abolished the ability of R. tropici to utilize nitrate as a sole carbon source, whereas a mutation in glnD further downstream (in mutant strain ME245) did not have an obvious effect on nitrate utilization. A gene similar to the Salmonella typhimurium virulence gene mviN overlaps the 3' end of the R. tropici glnD homologue. A mutation in mviN had no effect on the ability of CIAT899 to induce chlorosis in bean plants. Therefore the glnD homologue, but not mviN, appears to be required for induction of chlorosis in plants by R. tropici strain CIAT899. A high nitrogen: carbon ratio in the rhizosphere of bean plants also prevented R. tropici from inducing chlorosis in bean plants. Mutations in either the glnD homologue or mviN had no significant effect on root nodule formation or acetylene reduction activity. A mutation in mviN eliminated motility in R. tropici. The sequence data, the inability of the glnD mutant to utilize nitrate, and the role of the R. tropici glnD gene in chlorosis induction in plants, a process that is nitrogen regulated, suggest that glnD plays a role in nitrogen sensing in R. tropici as its homologues do in other organisms.

The role of uridylyltransferase in the control of Klebsiella pneumoniae nif gene regulation

The glnD gene in enteric bacteria encodes a uridylyltransferase/uridylyl-removing enzyme which acts as the primary nitrogen sensor in the nitrogen regulation (Ntr) system. We have investigated the role of this enzyme in transcriptional regulation of nitrogen fixation genes in Klebsiella pneumoniae by cloning glnD from this organism and constructing a null mutant by insertional inactivation of the chromosomal gene using the omega interposon. K. pneumoniae glnD encodes a 102.3 kDa polypeptide which is highly homologous to the predicted products of both Escherichia coli glnD and Azotobacter vinelandii nfrX. The glnD-omega mutant was unable to uridylylate PII and was altered in adenylylation/deadenylylation of glutamine synthetase. Uridylyltransferase was required for derepression of ntr-regulated promoters such as glnAp2 and pnifL but was not involved in the nif-specific response to changes in nitrogen status mediated by the nifL product. We conclude that a separate, as yet uncharacterised, nitrogen control system may be responsible for nitrogen sensing by NifL.

Activation of the dephosphorylation of nitrogen regulator I-phosphate of Escherichia coli

The transcription of sigma 54 RNA polymerase-dependent nitrogen-regulated genes is activated by nitrogen regulator I (NRI)-phosphate. The kinase NRII is responsible for the phosphorylation of NRI. It has been shown that NRII also has the ability to dephosphorylate NRI-phosphate but only when PII is present at a concentration greatly in excess of that of NRII. We have now shown that glutamate enables PII to stimulate the dephosphorylation of NRI-phosphate when present in equimolar concentration with NRII. This effect of glutamate appears to be a backup control that becomes effective when the normal regulation of PII activity is disabled.

Mutational analysis of the bacterial signal-transducing protein kinase/phosphatase nitrogen regulator II (NRII or NtrB)

The signal-transducing kinase/phosphatase nitrogen regulator II (NRII or NtrB) is required for the efficient positive and negative regulation of glnA, encoding glutamine synthetase, and the Ntr regulon in response to the availability of ammonia. Alteration of highly conserved residues within the kinase/phosphatase domain of NRII revealed that the positive and negative regulatory functions of NRII could be genetically separated and that negative regulation by NRII did not require the highly conserved His-139, Glu-140, Asn-248, Asp-287, Gly-289, Gly-291, Gly-313, or Gly-315 residue. These mutations affected the positive regulatory function of NRII to various extents. Certain substitutions at codons 139 and 140 resulted in mutant NRII proteins that were transdominant negative regulators of glnA and the Ntr regulon even in the absence of nitrogen limitation. In addition, we examined three small deletions near the 3' end of the gene encoding NRII; these resulted in altered proteins that retained the negative regulatory function but were defective to various extents in the positive regulatory function. A truncated NRII protein missing the C-terminal 59 codons because of a nonsense mutation at codon 291 lacked entirely the positive regulatory function but was a negative regulator of glnA even in the absence of nitrogen limitation. Thus, we have identified both point and deletion mutations that convert NRII into a negative regulator of glnA and the Ntr regulon irrespective of the nitrogen status of the cell.

The product of the Klebsiella aerogenes nac (nitrogen assimilation control) gene is sufficient for activation of the hut operons and repression of the gdh operon

In Klebsiella aerogenes, the formation of a large number of enzymes responds to the quality and quantity of the nitrogen source provided in the growth medium, and this regulation requires the action of the nitrogen regulatory (NTR) system in every case known. Nitrogen regulation of several operons requires not only the NTR system, but also NAC, the product of the nac gene, raising the question of whether the role of NAC is to activate operons directly or by modifying the specificity of the NTR system. We isolated an insertion of the transposon Tn5tac1 which puts nac gene expression under the control of the IPTG-inducible tac promoter rather than the nitrogen-responsive nac promoter. When IPTG was present, cells carrying the tac-nac fusion activated NAC-dependent operons and repressed NAC-repressible operons independent of the nitrogen supply and even in the absence of an active NTR system. Thus, NAC is sufficient to regulate operons like hut (encoding histidase) and gdh (encoding glutamate dehydrogenase), confirming the model that the NTR system activates nac expression and NAC activates hut and represses gdh. Activation of urease formation occurred at a lower level of NAC than that required for glutamate dehydrogenase repression, and activation of histidase formation required still more NAC.

Characterization of Escherichia coli glnL mutations affecting nitrogen regulation

Nitrogen regulator II (NRII), the product of the Escherichia coli glnL (ntrB) gene, regulates the activation of transcription of glnA and the Ntr regulon by catalyzing the phosphorylation and dephosphorylation of the transcription factor NRI. Previous results have indicated that under conditions of nitrogen excess, transcriptional activation is prevented by an NRI-phosphate phosphatase activity that is observed when NRII and another signal transduction protein known as PII (the glnB product) interact. The availability of PII for this interaction is controlled by a uridylytransferase/uridylyl-removing enzyme, encoded by glnD, that reversibly modifies PII in response to intracellular signals of nitrogen availability. Here we describe the isolation and characterization of missense mutations in glnL that suppress the Ntr- phenotype resulting from a leaky glnD mutation. The regulation of glnA expression in the pseudorevertants was found to vary from complete insensitivity to ammonia in some strains (GlnC phenotype) to nearly normal regulation by ammonia in other strains. Sequence analysis indicated that in 16 instances suppression was due to point mutations at 14 different sites; 10 different mutations resulting in a variety of phenotypes were identified in a cluster extending from codons 111 to 154 flanking the site of NRII autophosphorylation at His-139. Complementation experiments with multicopy plasmids encoding NRII or PII showed that suppression by GlnC glnL alleles was eliminated upon introduction of the plasmid encoding NRII but was not affected by introduction of the plasmid encoding PII. Conversely, suppression by certain glnL alleles that resulted in regulated expression of glnA was eliminated upon introduction of either the plasmid encoding NRII or that encoding PII. We hypothesize that mutants of the latter type result in a subtle perturbation of the NRII-PII interaction and suggest two possible mechanisms for their effects.

The product of the nitrogen fixation regulatory gene nfrX of Azotobacter vinelandii is functionally and structurally homologous to the uridylyltransferase encoded by glnD in enteric bacteria

We sequenced the nitrogen fixation regulatory gene nfrX from Azotobacter vinelandii, mutations in which cause a Nif- phenotype, and found that it encodes a 105-kDa protein (NfrX), the N terminus of which is highly homologous to that of the uridylyltransferase-uridylyl-removing enzyme encoded by glnD in Escherichia coli. In vivo complementation experiments demonstrate that the glnD and nfrX products are functionally interchangeable. A vinelandii nfrX thus appears to encode a uridylyltransferase-uridylyl-removing enzyme, and in this paper we report the first sequence of such a protein. The Nif- phenotype of nfrX mutants can be suppressed by a second mutation in a recently identified nifL-like gene immediately upstream of nifA in A. vinelandii. NifL mediates nif regulation in response to the N status in A. vinelandii, presumably by inhibiting NifA activator function as occurs in Klebsiella pneumoniae; thus, one role of NfrX is to modify, either directly or indirectly, the activity of the nifL product.

Complementation of a pleiotropic Nif-Gln regulatory mutant of Rhodospirillum rubrum by a previously unrecognized Azotobacter vinelandii regulatory locus

A spontaneous pleiotropic Nif- mutation in Rhodospirillum rubrum has been partially characterized biochemically and by complementation analysis with recombinant plasmids carrying Azotobacter vinelandii DNA in the vicinity of ORF12 [Jacobson et al. (1989) J. Bacteriol 171: 1017-1027]. In addition to being unable to grow on N2 as a nitrogen source the phenotypic characterization of this and other metronidazole enriched spontaneous mutants showed (a) no nitrogenase activity, (b) the absence of NifHDK polypeptides, (c) a slower growth rate on NH4+, (d) approximately 50% higher glutamine synthetase (GS) activity than the wild-type, which was repressible, (e) an inability to switch-off GS activity in response to an NH4+ up-shift, and (f) an inability to modify (32P-label) the GS polypeptide. The apparent relationship between the absence of nifHDK expression and the absence of GS adenylylation cannot be explained in terms of the current model for nif gene regulation. However, R. rubrum transconjugants receiving A. vinelandii DNA which originated immediately upstream from nifH, restored all aspects of the wild-type phenotype. These data suggest a here-to-fore unrecognized relationship between nif expression and GS switch-off (adenylylation) activity, and the existence of a previously unidentified regulatory locus in Azotobacter that complements this mutation.

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.

Cloning and characterization of the glnA gene of Azospirillum brasilense Sp7

A plasmid which, by complementation, restored a Gln+Nif+ phenotype to the Gln-Nif- Azospirillum brasilense mutant 7029, was isolated from a gene bank of total DNA of A. brasilense Sp7 (ATCC 29145) constructed in the broad host range vector pVK100. This plasmid contained the structural gene (glnA) for glutamine synthetase. The glnA gene was mapped by Tn5 insertion and DNA hybridization with a Klebsiella pneumoniae glnA probe. The direction of transcription of glnA was determined. The glnA product was identified as a 50-Kd polypeptide which could be adenylylated in Escherichia coli, and glutamine synthetase activity was characterized in E. coli. Plasmids containing the glnA gene restored glutamine-independent growth and a Nif+ phenotype to Gln-Nif- and Gln-Nifc mutants of Azospirillum.

Role of glnB and glnD gene products in regulation of the glnALG operon of Escherichia coli

We have isolated insertion and deletion mutants in glnB, the structural gene of PII, a member of the adenylylation system for glutamine synthetase of Escherichia coli, to study the role of PII in the regulation of the synthesis of glutamine synthetase and of histidase in response to nitrogen deprivation or excess. We have studied the effects of this mutation alone and combined with null mutations resulting from the insertion of transposons or from a deletion in the other genes affecting this regulation, glnD, glnF (ntrA), glnG (ntrC), and glnL (ntrB). Our results confirm that only the products of glnF and glnG are essential for this regulation. In cells of the wild type, the response is mediated by the products of glnD and glnB via the product of glnL. In the condition of nitrogen excess, PII, the product of glnB, appears to convert the product of glnL to a form that prevents the activation of transcription of the structural genes for glutamine synthetase and for histidase by the products of glnF and glnG. During nitrogen deprivation, uridylyltransferase, the product of glnD, is activated by the intracellular excess of 2-ketoglutarate over glutamine and converts PII to PII-UMP and changes the form of the glnL product to one that stimulates the activation of transcription of glutamine synthetase and histidase by the products of glnF and glnG.

Bacillus subtilis glutamine synthetase mutants pleiotropically altered in glucose catabolite repression

Strain SF22, a glutamine-requiring (Gln-) mutant of Bacillus subtilis SMY, is likely to have a mutation in the structural gene for glutamine synthetase, since this strain synthesized 22 to 55% as much glutamine synthetase antigen as did wild-type cells in a 10-min period but had less than 3% of wild-type glutamine synthetase enzymatic activity. The expression of several genes subject to glucose catabolite repression was altered in the Gln- mutant. The induced levels of alpha-glucosidase, histidase, and aconitase were 3.5- to 4-fold higher in SF22 cells than in wild-type cells grown in glucose-glutamine medium, and citrate synthase levels were 8-fold higher in the Gln- mutant than in wild-type cells. The relief of glucose catabolite repression in the Gln- mutant may result from poor utilization of glucose. Examination of the intracellular metabolite pools of cells grown in glucose-glutamine medium showed that the glucose-6-phosphate pool was 2.5-fold lower, the pyruvate pool was 4-fold lower, and the 2-ketoglutarate pool was 2.5-fold lower in the Gln- cells than they were in wild-type cells. Intracellular levels of glutamine were sixfold higher in the Gln- mutant than in wild-type cells. Measurements of enzymes involved in glutamine transport and utilization showed that the elevated pools of glutamine in the Gln- mutant resulted from a threefold increase in glutamine permease and a fivefold decrease in glutamate synthase. The pleiotropic effect of the gln-22 mutation on the expression of several genes suggests that either the glutamine synthetase protein or its enzymatic product, glutamine, is involved in the regulation of several metabolic pathways in B. subtilis.

Activation of Klebsiella pneumoniae and Rhizobium meliloti nitrogenase promoters by gln (ntr) regulatory proteins

We have studied the expression, in different Escherichia coli gln (ntr) mutants, of fusions (constructed in vitro) of the nifHDK (nitrogenase) promoters from Klebsiella pneumoniae and Rhizobium meliloti to E. coli lacZ. Derepression of the K. pneumoniae nifH::lacZ fusion requires the glnF (ntrA) gene product in addition to the K. pneumoniae nifA gene product, indicating that regulation of the K. pneumoniae nif genes is more closely integrated with the overall nitrogen control system than previously demonstrated. Derepression of the R. meliloti nifH::lacZ fusion in E. coli by the K. pneumoniae nifA gene product (which we had previously shown) exhibits the same requirement for glnF. Derepression of the R. meliloti nifH::lacZ fusion, but not the K. pneumoniae nifH::lacZ fusion, can be mediated by the glnG (ntrC) gene product, suggesting that the gln regulatory genes might directly regulate the symbiotic nitrogen fixation genes in Rhizobium.

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.

Fine-structure deletion map and complementation analysis of the glnA-glnL-glnG region in Escherichia coli

A total of 399 independent mutants of Escherichia coli were obtained which have point and insertion mutations in the glnA region. Mutants isolated included Gln- and Reg- strains (unable to utilize arginine as a nitrogen source). Mutations were mapped with 73 deletion-containing derivatives of a lambda gln phage. Complementation analysis was performed with lambda gln derivatives containing point mutations which conferred a Gln- or Reg- phenotype. Deletion mapping and complementation analysis assigned 104 mutations in 24 deletion intervals to glnA. Mutations in Reg- strains were assigned to two genes, glnL and glnG. glnL contained 131 mutations in 12 deletion intervals, and glnG contained 164 mutations in 10 deletion intervals. The gene order is glnA-glnL-glnG, transcribed from left to right. Polarity of insertion mutations indicates that glnL and glnG form from left to right. Polarity of insertion mutations indicates that glnL and glnG form an operon. Complementation analysis of glnA insertion mutations with glnL and glnG mutations showed polarity of glnA onto most glnL and glnG alleles, suggesting that transcription of glnA may proceed into the glnL-glnG operon. All mutations analyzed in glnA conferred a Gln- phenotype. However, we also found that over half of the Gln- strains isolated ater chemical mutagenesis contained point mutations in glnG. Mutants which synthesized a high level of glutamine synthetase in the presence of ammonia (GlnC phenotype) were selected as revertants of a strain with a Tn10 insertion in glnD and were mapped with chromosomal deletions. Results indicate that mutations in 12 and 15 examined strains clearly map outside of glnA, probably in glnL.

Role of glnA-linked genes in regulation of glutamine synthetase and histidase formation in Klebsiella aerogenes

We isolated mutants of Klebsiella aerogenes with insertions of transposon Tn5 linked to the structural gene for glutamine synthetase, glnA. We found that K. aerogenes, like Escherichia coli and Salmonella typhimurium, contains a regulatory gene, glnG, which is closely linked to but distinct from glnA. The product of glnG is essential for normal regulation of the synthesis of glutamine synthetase and histidase. We analyzed two mutations which affected the regulation of the formation of these enzymes and appeared to be outside glnG. The results of our studies suggest the presence in K. aerogenes of at least two regulatory genes located in the glnA region, namely, glnG and another gene, the transcription of which can be initiated at the promoter of glnA.

Characterization of a gene, glnL, the product of which is involved in the regulation of nitrogen utilization in Escherichia coli

DNA was prepared from a strain of Escherichia coli bearing a mutation which confers the GlnC phenotype (inability to reduce the expression of glnA and other nitrogen-regulated operons in response to ammonia in the growth medium). A fragment of this DNA carrying glnA, the structural gene for glutamine synthetase, was cloned on plasmid pBR322. By using recombination in vitro, we mapped the GlnC mutation to a region between glnA and glnG. This region defines a gene, glnL, which codes for a trans-acting product; the GlnC mutant produces an altered product. The glnL product plays a key role in the communication of information concerning the quality and abundance of the nitrogen source in the growth medium to a destination responsible for the regulation of glnA and other genes for enzymes responsible for nitrogen utilization.

Regulation of glutamine synthetase by regulatory protein PII in Klebsiella aerogenes mutants lacking adenylyltransferase

A mutation of Klebsiella aerogenes causing production of an altered PII regulatory protein which stimulates overadenylylation of glutamine synthetase and also prevents its derepression was combined with mutations abolishing the activity of adenylyltransferase. The results support the idea that PII plays a role in the regulation of the level of glutamine synthetase which is independent of its interaction with adenylyltransferase.

Serine utilization by Klebsiella aerogenes

Klebsiella aerogenes was found to contain a specific L-serine dehydrase that was induced by threonine, glycine or leucine, but not by its substrate. Cellular concentrations were sensitive to carbon rather than nitrogen sources in the growth medium. A nonspecific isoleucine-sensitive L-threonine dehydrase supplemented the specific L-serine dehydrase activity. K. aerogenes also contains a leucine-inducible L-threonine dehydrogenase which probably initiated a threonine-utilization pathway in which the serine-specific dehydrate participated. Strains that were altered in their ability to metabolize serine differed in either L-serine dehydrase or L-threonine dehydrase activity. Thus, K. aerogenes growing on L-serine as a sole nitrogen source relies upon two enzymes that metabolize the amino acid as subsidiary functions.

Regulation of the synthesis of glutamine synthetase by the PII protein in Klebsiella aerogenes

Certain mutations at the glaB locus result in the failure to fully derepress glutamine synthetase [L-glutamate:ammonia ligase (ADP-forming), EC 6.3.1.2] and to convert it to the active nonadenylylated form in response to nitrogen limitation. In these mutants the PII regulatory protein is altered such that it cannot be converted by uridylyltransferase to the form stimulating deadenylylation of glutamine synthetase by adenylyltransferase. Additional mutations as well as insertions of transposon Tn5 at the glnB site result in the loss of PII. The loss of PII does not prevent adenylylation and deadenylylation of glutamine synthetase but reduces the rates of these reactions. Cells lacking PII have a high level of glutamine synthetase even when they are grown with an excess of ammonia and the enzyme is highly adenylylated. The results suggest that the PII protein plays a role, independent of its effect on adenylylation, in the regulation of the level of glutamine synthetase.

Isolation of Klebsiella aerogenes mutants cis-dominant for glutamine synthetase expression

We have isolated three strains of Klebsiella aerogenes that failed to show repression of glutamine synthetase even when grown under the most repressing conditions for the wild-type strain. These mutant strains were selected as glutamine-independent derivatives of a strain that is merodiploid for the glnA region and contains a mutated glnF allele. The mutation responsible for the Gln+ phenotype in each strain was tightly linked to glnA, the structural gene for glutamine synthetase, and was dominant to the wild-type allele. These mutations are probably lesions in the control region of the glnA gene, since each mutation was cis-dominant for constitutive expression of the enzyme in hybrid merodiploid strains. Strains harboring this class of mutations were unable to produce a high level of glutamine synthetase unless they also contained an intact glnF gene, and unless cells were grown in derepressing medium. This study supports the idea that the glnA gene is regulated both positively and negatively, and that the deoxyribonucleic acid sites critical for positive control and negative control are functionally distinct.

Regulation of glutamine synthetase formation in Escherichia coli: characterization of mutants lacking the uridylyltransferase

A lambda phage (lambdaNK55) carrying the translocatable element Tn10, conferring tetracycline resistance (Tetr), has been utilized to isolate glutamine auxotrophs of Escherichia coli K-12. Such strains lack uridylyltransferase as a result of an insertion of the TN10 element in the glnD gene. The glnD::Tn10 insertion has been mapped at min 4 on the E. coli chromosome and 98% contransducible by phage P1 with dapD. A lambda transducing phage carrying the glnD gene has been identified. A glnD::Tn10 strain synthesizes highly adenylylated glutamine synthetase under all conditions of growth and fails to accumulate high levels of glutamine synthetase in response to nitrogen limitation. However, this strain, under nitrogen-limiting conditions, allows synthesis of 10 to 20 milliunits of biosynthetically active glutamine synthetase per mg of protein, which is sufficient to allow slow growth in the absence of glutamine. The GlnD phenotype in E. coli can be suppressed by the presence of mutations which increase the quantity of biosynthetically active glutamine synthetase.