Chapter 5 : Control of Nitrogen Assimilation by the NRI-NRII Two-Component System of Enteric Bacteria

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.

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) .