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Category: Microbial Genetics and Molecular Biology
Control of Nitrogen Assimilation by the NRI-NRII Two-Component System of Enteric Bacteria, Page 1 of 2
< Previous page | Next page > /docserver/preview/fulltext/10.1128/9781555818319/9781555810894_Chap05-1.gif /docserver/preview/fulltext/10.1128/9781555818319/9781555810894_Chap05-2.gifAbstract:
Enteric bacteria such as Escherichia coli, Salmonella typhimurium, and their relatives regulate the expression of glutamine synthetase (GS) and other enzymes important in nitrogen assimilation in response to changes in the availability of nitrogen. In this review, the current state of knowledge about the mechanisms of signal transduction by NRI and NRII is summarized briefly. Escherichia coli and related bacteria precisely regulate the level of GS activity by three distinct mechanisms. First, the intracellular concentration of the enzyme is regulated in response to the intracellular nitrogen status. Second, the activity of the enzyme is regulated by reversible covalent modification. Finally, the activity of GS is allosterically controlled by cumulative feedback inhibition by eight small molecules: tryptophan, histidine, carbamyl phosphate, glucosamine-6-phosphate, CTP, AMP, alanine, and glycine. A hypothesis to explain the different phenotypes resulting from the suppressor mutations in glnL is as follows: the suppressors resulting in the constitutive expression of glnA are likely to have either eliminated the capacity of NRII to interact with PII or rendered this interaction unproductive in bringing about the regulated phosphatase activity. The authors examined the ability of partially modified PII to elicit the regulated phosphatase activity and found that it was partially active. They also examined the ability of immobilized NRII to retain PII, using a column chromatography method.
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Signal transduction system controlling activity of glutamine synthetase and transcription of the Ntr regulon. Abbreviations: GLN, glutamine; 2KG, 2-ketoglutarate. The small molecules activating reactions are shown without boxes, and the small molecules inhibiting reactions are shown in boxes. The figure is similar to Fig. 1 of Atkinson et al. (1994) and Kamberov et al. (1994a , submitted).
Signal transduction system controlling activity of glutamine synthetase and transcription of the Ntr regulon. Abbreviations: GLN, glutamine; 2KG, 2-ketoglutarate. The small molecules activating reactions are shown without boxes, and the small molecules inhibiting reactions are shown in boxes. The figure is similar to Fig. 1 of Atkinson et al. (1994) and Kamberov et al. (1994a , submitted).
Composite nitrogen regulatory cascade from Escherichia coli, Klebsiella aerogenes, and Klebsiella pneumoniae. In this figure, the activation of nitrogen-regulated genes is depicted as a developmental pathway. Only a few examples of nitrogen-regulated genes and operons are shown. At the top of the cascade is the glnALG operon (glnA ntrBC operon), which is activated by a low intracellular concentration of NRI∼P. NRII and/or acetyl phosphate can give rise to NRI∼P by phosphotransfer to NRI. NRII + PII result in the destruction of NRI∼P (regulated phosphatase activity). One product of the glnALG operon is GS. Another result of the activation of the glnALG operon is an increase in the intracellular concentration of NRI. An elevated intracellular concentration of NRI∼P results in the activation of genes and operons at the second level of the cascade. For example, the glnH (glutamine transport) and aut (arginine use) genes of E. coli are among the Ntr operons so controlled. In other cases, the elevated intracellular concentration of NRI results in the activation of genes encoding transcription factors. The Klebsiella aerogenes nac gene product, NAC, activates put (proline use), hut (histidine use), and genes encoding urease and represses its own expression and the expression of gdh (glutamate dehydrogenase). The nifLA operon of K. pneumoniae encodes the activator of nif gene expression, NifA, and a regulator of NifA activity, NifL. NifA activates transcription of the nif genes, which encode nitrogenase and associated proteins required for the assimilation of N2.
Composite nitrogen regulatory cascade from Escherichia coli, Klebsiella aerogenes, and Klebsiella pneumoniae. In this figure, the activation of nitrogen-regulated genes is depicted as a developmental pathway. Only a few examples of nitrogen-regulated genes and operons are shown. At the top of the cascade is the glnALG operon (glnA ntrBC operon), which is activated by a low intracellular concentration of NRI∼P. NRII and/or acetyl phosphate can give rise to NRI∼P by phosphotransfer to NRI. NRII + PII result in the destruction of NRI∼P (regulated phosphatase activity). One product of the glnALG operon is GS. Another result of the activation of the glnALG operon is an increase in the intracellular concentration of NRI. An elevated intracellular concentration of NRI∼P results in the activation of genes and operons at the second level of the cascade. For example, the glnH (glutamine transport) and aut (arginine use) genes of E. coli are among the Ntr operons so controlled. In other cases, the elevated intracellular concentration of NRI results in the activation of genes encoding transcription factors. The Klebsiella aerogenes nac gene product, NAC, activates put (proline use), hut (histidine use), and genes encoding urease and represses its own expression and the expression of gdh (glutamate dehydrogenase). The nifLA operon of K. pneumoniae encodes the activator of nif gene expression, NifA, and a regulator of NifA activity, NifL. NifA activates transcription of the nif genes, which encode nitrogenase and associated proteins required for the assimilation of N2.
Pathways for the formation and breakdown of acetyl phosphate. The pta gene encodes the enzyme phosphotransacetylase, and the ackA gene encodes the enzyme acetate kinase.
Pathways for the formation and breakdown of acetyl phosphate. The pta gene encodes the enzyme phosphotransacetylase, and the ackA gene encodes the enzyme acetate kinase.
Mutations in glnL (ntrB), encoding NRII. (A) Schematic depiction of the NRn protein. The nonconserved N-terminal domain is shown as a thin line, and the conserved histidine kinase-phosphatase domain is shown as a thick line. Within this conserved domain, three highly conserved regions are found, depicted with crosshatching and referred to here as region 1, region 2, and region 3. The consensus sequence for the highly conserved regions are shown. The standard single letter amino acid code is used with the following exceptions: X refers to positions where at least 50% of the family have a nonpolar amino acid (I, L, M, or V), Z refers to positions where at least 50% of the family have a polar amino acid (A, G, P, S, or T),J refers to positions where at least 50% of the family have a basic amino acid (H, K, or R), and O refers to positions where at least 50% of the family have an acidic or amidic amino acid (D, E, N, or Q). Positions with less than 50% conservation among the kinase family are shown by dashes. Positions at which mutations were introduced by site-specific mutagenesis are indicated by arrowheads. For a more complete description of the conservation in the kinase family, see Parkinson and Kofoid (1992) . Adapted from Fig. 5 of Parkinson and Kofoid (1992) with permission and has also appeared in Atkinson and Ninfa (1993) . (B) Mutational analysis of the highly conserved regions of NRII. The identities of the alterations made are shown. The phenotypes resulting from these changes are summarized in the text and in Atkinson and Ninfa (1993) . (C) Alterations in NRn resulting from the introduction of nonsense codons or small deletions, ter refers to termination codons. 307 refers to the deletion of codon 307, while 307–311 refers to deletion of codons 307 to 311. (D) Mutations selected as suppressors of the Ntr- phenotype resulting from the glnD99::Tn10 mutation. B, C, and D are reproduced from Fig. 2 of Atkinson and Ninfa (1993) .
Mutations in glnL (ntrB), encoding NRII. (A) Schematic depiction of the NRn protein. The nonconserved N-terminal domain is shown as a thin line, and the conserved histidine kinase-phosphatase domain is shown as a thick line. Within this conserved domain, three highly conserved regions are found, depicted with crosshatching and referred to here as region 1, region 2, and region 3. The consensus sequence for the highly conserved regions are shown. The standard single letter amino acid code is used with the following exceptions: X refers to positions where at least 50% of the family have a nonpolar amino acid (I, L, M, or V), Z refers to positions where at least 50% of the family have a polar amino acid (A, G, P, S, or T),J refers to positions where at least 50% of the family have a basic amino acid (H, K, or R), and O refers to positions where at least 50% of the family have an acidic or amidic amino acid (D, E, N, or Q). Positions with less than 50% conservation among the kinase family are shown by dashes. Positions at which mutations were introduced by site-specific mutagenesis are indicated by arrowheads. For a more complete description of the conservation in the kinase family, see Parkinson and Kofoid (1992) . Adapted from Fig. 5 of Parkinson and Kofoid (1992) with permission and has also appeared in Atkinson and Ninfa (1993) . (B) Mutational analysis of the highly conserved regions of NRII. The identities of the alterations made are shown. The phenotypes resulting from these changes are summarized in the text and in Atkinson and Ninfa (1993) . (C) Alterations in NRn resulting from the introduction of nonsense codons or small deletions, ter refers to termination codons. 307 refers to the deletion of codon 307, while 307–311 refers to deletion of codons 307 to 311. (D) Mutations selected as suppressors of the Ntr- phenotype resulting from the glnD99::Tn10 mutation. B, C, and D are reproduced from Fig. 2 of Atkinson and Ninfa (1993) .