Regulation of autophosphorylation of Escherichia coli nitrogen regulator II by the PII signal transduction protein.
The nitrogen regulator II (NRII or NtrB)-NRI (NtrC) two-component signal transduction system regulates the transcription of nitrogen-regulated genes in Escherichia coli. The NRII protein has both kinase and phosphatase activities and catalyzes the phosphorylation and dephosphorylation of NRI, which activates transcription when phosphorylated. The phosphatase activity of NRII is activated by the PII signal transduction protein. We showed that PII was also an inhibitor of the kinase activity of NRII. The data were consistent with the hypothesis that the kinase and phosphatase activities of two-component system kinase/phosphatase proteins are coordinately and reciprocally regulated. The ability of PII to regulate NRII is allosterically controlled by the small-molecule effector 2-ketoglutarate, which binds to PII. We studied the effect of 2-ketoglutarate on the regulation of the kinase and phosphatase activities of NRII by PII, using a coupled enzyme system to measure the rate of cleavage of ATP by NRII. The data were consistent with the following hypothesis: when not complexed with 2-ketoglutarate, PII cannot bind to NRII and has no effect on its competing NRI kinase and phosphatase activities. Under these conditions, the kinase activity of NRII is dominant. At low 2-ketoglutarate concentrations, PII trimers complexed with a single molecule of 2-ketoglutarate interact with NRII to inhibit its kinase activity and activate its phosphatase activity. However, at high 2-ketoglutarate concentrations, PII binds additional ligand molecules and is rendered incapable of binding to NRII, thereby releasing inhibition of NRII's kinase activity and effectively inhibiting its phosphatase activity (by failing to stimulate it). (+info)
Role of ArgR in activation of the ast operon, encoding enzymes of the arginine succinyltransferase pathway in Salmonella typhimurium.
The ast operon, encoding enzymes of the arginine succinyltransferase (AST) pathway, was cloned from Salmonella typhimurium, and the nucleotide sequence for the upstream flanking region was determined. The control region contains several regulatory consensus sequences, including binding sites for NtrC, cyclic AMP receptor protein (CRP), and ArgR. The results of DNase I footprintings and gel retardation experiments confirm binding of these regulatory proteins to the identified sites. Exogenous arginine induced AST under nitrogen-limiting conditions, and this induction was abolished in an argR derivative. AST was also induced under carbon starvation conditions; this induction required functional CRP as well as functional ArgR. The combined data are consistent with the hypothesis that binding of one or more ArgR molecules to a region between the upstream binding sites for NtrC and CRP and two putative promoters plays a pivotal role in modulating expression of the ast operon in response to nitrogen or carbon limitation. (+info)
Azorhizobium caulinodans PII and GlnK proteins control nitrogen fixation and ammonia assimilation.
We herein report that Azorhizobium caulinodans PII and GlnK are not necessary for glutamine synthetase (GS) adenylylation whereas both proteins are required for complete GS deadenylylation. The disruption of both glnB and glnK resulted in a high level of GS adenylylation under the condition of nitrogen fixation, leading to ammonium excretion in the free-living state. PII and GlnK also controlled nif gene expression because NifA activated nifH transcription and nitrogenase activity was derepressed in glnB glnK double mutants, but not in wild-type bacteria, grown in the presence of ammonia. (+info)
The global nitrogen regulator NtcA regulates transcription of the signal transducer PII (GlnB) and influences its phosphorylation level in response to nitrogen and carbon supplies in the Cyanobacterium synechococcus sp. strain PCC 7942.
The PII protein is encoded by a unique glnB gene in Synechococcus sp. strain PCC 7942. Its expression has been analyzed in the wild type and in NtcA-null mutant cells grown under different conditions of nitrogen and carbon supply. RNA-DNA hybridization experiments revealed the presence of one transcript species 680 nucleotides long, whatever the nutrient conditions tested. A second transcript species, 620 nucleotides long, absent in the NtcA null mutant, was observed in wild-type cells that were nitrogen starved for 2 h under both high and low CO2 and in the presence of nitrate under a high CO2 concentration. Primer extension analysis indicated that the two transcript species are generated from two tandem promoters, a sigma70 Escherichia coli-type promoter and an NtcA-dependent promoter, located 120 and 53 nucleotides, respectively, from the glnB initiation codon. The NtcA-dependent promoter is up-regulated under the conditions mentioned above, while the sigma70 E. coli-type promoter displays constitutive levels of transcripts in the NtcA null mutant and slightly different levels in the wild-type cells, depending on the nitrogen and carbon supplies. In general, a good correlation between the amounts of the two transcript species and that of the PII protein was observed, as revealed by immunodetection with specific antibodies. The phosphorylation level of PII in the wild type is inversely correlated with nitrogen availability and directly correlated with higher CO2 concentration. This regulation is correspondingly less stringent in the NtcA null mutant cells. In contrast, the dephosphorylation of PII is NtcA independent. (+info)
Physical evidence for a phosphorylation-dependent conformational change in the enhancer-binding protein NtrC.
The bacterial enhancer-binding protein nitrogen regulatory protein C (NtrC) activates transcription by sigma54-containing RNA polymerase in a reaction that depends on ATP hydrolysis. Phosphorylation of an aspartate residue in the N-terminal receiver domain of NtrC induces oligomerization of the protein and activates the ATPase activity, which is a function of its central output domain. To study the role of the receiver domain of NtrC, which is known to act positively, we isolated mutant forms of the protein carrying single cysteine residues and derivatized them with a sulfhydryl-specific nitroxide reagent for electron paramagnetic resonance studies. Single cysteines were placed at four positions at which we had obtained constitutive amino acid substitutions, those that yield activity without phosphorylation. In only one case, derivatized C86 in alpha-helix 4 of the receiver domain, did the motion of the side chain become dramatically slower upon phosphorylation. Importantly, derivatized NtrCD86C (NtrCD86C*) activated transcription normally. Additional experiments indicated that the spectral change observed upon phosphorylation of NtrCD86C* was due to interdomain interactions rather than a conformational change within the N-terminal domain itself. These interactions did not appear to occur within a monomer. Although it is not clear whether the spectral change seen upon phosphorylation of NtrCD86C* is due to an interaction that occurs within a dimer of NtrC or requires the formation of higher-order oligomers, the change indicated that alpha-helix 4 of the receiver domain probably plays an important role in communication with the remainder of the protein. (+info)
Interaction of Azospirillum lipoferum with wheat germ agglutinin stimulates nitrogen fixation.
In vitro, the nitrogen fixation capability of A. lipoferum is efficiently increased in the presence of wheat germ agglutinin (WGA). A putative WGA-binding receptor, a 32-kDa protein, was detected in the cell capsule. The stimulatory effect required N-acetyl-D-glucosamine dimer (GlcNAcdi) terminated sugar side chains of the receptor and was dependent on the number of GlcNAcdi links involved in receptor-WGA interface. Binding to the primary sugar binding sites on WGA had a larger stimulatory effect than binding to the secondary sites. The WGA-receptor complex generated stimulus led to elevated transcription of the nifH and nifA genes and of the glnBA gene cluster but not of the glnA gene from its own promoter. There may well be a signalling cascade contributing to the regulation of nitrogen fixation. (+info)
MgATP binding and hydrolysis determinants of NtrC, a bacterial enhancer-binding protein.
When phosphorylated, the dimeric form of nitrogen regulatory protein C (NtrC) of Salmonella typhimurium forms a larger oligomer(s) that can hydrolyze ATP and hence activate transcription by the sigma(54)-holoenzyme form of RNA polymerase. Studies of Mg-nucleoside triphosphate binding using a filter-binding assay indicated that phosphorylation is not required for nucleotide binding but probably controls nucleotide hydrolysis per se. Studies of binding by isothermal titration calorimetry indicated that the apparent K(d) of unphosphorylated NtrC for MgATPgammaS is 100 microM at 25 degrees C, and studies by filter binding indicated that the concentration of MgATP required for half-maximal binding is 130 microM at 37 degrees C. Filter-binding studies with mutant forms of NtrC defective in ATP hydrolysis implicated two regions of its central domain directly in nucleotide binding and three additional regions in hydrolysis. All five are highly conserved among activators of sigma(54)-holoenzyme. Regions implicated in binding are the Walker A motif and the region around residues G355 to R358, which may interact with the nucleotide base. Regions implicated in nucleotide hydrolysis are residues S207 and E208, which have been proposed to lie in a region analogous to the switch I effector region of p21(ras) and other purine nucleotide-binding proteins; residue R294, which may be a catalytic residue; and residue D239, which is the conserved aspartate in the putative Walker B motif. D239 appears to play a role in binding the divalent cation essential for nucleotide hydrolysis. Electron paramagnetic resonance analysis of Mn(2+) binding indicated that the central domain of NtrC does not bind divalent cation strongly in the absence of nucleotide. (+info)
Genetics of nitrogen regulation in Methanococcus maripaludis.
We have used genetic methods in Methanococcus maripaludis to study nitrogen metabolism and its regulation. We present evidence for a "nitrogen regulon" in Methanococcus and Methanobacterium species containing genes of nitrogen metabolism that are regulated coordinately at the transcriptional level via a common repressor binding site sequence, or operator. The implied mechanism for regulation resembles the general bacterial paradigm for repression, but contrasts with well-known mechanisms of nitrogen regulation in bacteria, which occur by activation. Genes in the nitrogen regulons include those for nitrogen fixation, glutamine synthetase, (methyl)ammonia transport, the regulatory protein GlnB, and ammonia-dependent NAD synthetase, as well as a gene of unknown function. We also studied the function of two novel GlnB homologues that are encoded within the nif gene cluster of diazotrophic methanogens. The phenotype resulting from a glnB null mutation in M. maripaludis provides direct evidence that glnB-like genes are involved in "ammonia switch-off," the post-transcriptional inhibition of nitrogen fixation upon addition of ammonia. Finally, we show that the gene nifX is not required for nitrogen fixation, in agreement with findings in several bacteria. These studies illustrate the utility of genetic methods in M. maripaludis and show the enhanced perspective that studies in the Archaea can bring to known biological systems. (+info)