Recombinant Escherichia coli Nitrate/nitrite sensor protein narX (narX)
Description
Introduction
The NarX protein, a histidine kinase receptor, plays a crucial role in the regulation of anaerobic respiration in various bacteria by responding to nitrate and nitrite . In Escherichia coli, NarX, along with NarQ, constitutes a two-component regulatory system that modulates the expression of anaerobic respiratory genes in response to nitrate and nitrite . These sensor-transmitter proteins detect the presence of nitrate in the cell environment and transmit this signal to the response regulator, NarL . Upon activation, NarL binds to DNA and modulates the expression of its target genes through repression or activation of transcription .
Regulatory and Catalytic Domains
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Regulatory Domain The periplasmic and transmembrane regions of NarX are involved in signal recognition . Removal of the regulatory domain results in a constitutive or "locked-on" kinase .
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Catalytic Domain The cytoplasmic region of NarX contains the ATP-dependent autokinase activity .
Autophosphorylation and Dephosphorylation
NarX autophosphorylates in response to nitrate and other signals . The kinetic values for autophosphorylation and dephosphorylation are shown in Table 1 .
Table 1: Kinetic Values for Autophosphorylation and Dephosphorylation
| Protein | Kinetic value b | Reference |
|---|---|---|
| Autophosphorylation | Dephosphorylation | |
| K<sub>m</sub> ATP (μM) | k (10<sup>-4</sup>) (s<sup>-1</sup>) | |
| MBP-NarX 185 a | 2.4 ± 0.7 | 0.5 ± 0.2 |
| MBP-NarQ 182 a | 22.8 ± 9.3 | 2.2 ± 1.1 |
| CheA | 300 ± 75 | 260 ± 40 |
| NR II (NtrB) | 31 ± 1.4 | 118 ± 2.5 |
| KinA | 74 ± 11 | 19 ± 5 |
| EnvZ | 218 | 0.81 c |
| PhoQ | 20.1 ± 13.1 | − d |
Role in Nitrate Regulation
NarX functions independently of NarQ in vivo to activate NarL when nitrate is present and inactivate the NarL protein when no nitrate is available to the cell . These in vivo results align with the documented kinase and phosphatase activities of both NarQ and NarX in vitro .
Mutational Analysis
Mutations in the amino-terminal coding region of NarX can confer an altered signaling phenotype . The amino terminus of the NarX protein is important for the differential response to nitrate and nitrite .
Product Specs
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Target Background
NarX functions as a sensor for nitrate/nitrite, transmitting signals indicating nitrate availability to the NarL protein and both nitrate/nitrite availability to the NarP protein. In the presence of nitrate, NarX likely activates NarL and NarP through phosphorylation. Conversely, NarX may negatively regulate NarL activity through dephosphorylation in the absence of nitrate.
- Bacterial adenylate cyclase two-hybrid analysis revealed strong interactions between NarX-NarL, NarQ-NarL, and NarQ-NarP, but a significantly weaker interaction between NarX-NarP. PMID: 25873583
- Missense mutations in NarX impact its phosphatase function without directly altering the active site. PMID: 23517441
- The HAMP domain regions HD2 and S-helix overlap, forming a continuous coiled-coil structure characterized by a heptad repeat stutter. Deletions within the conserved S-helix core, particularly those removing the heptad stutter, resulted in impaired induction phenotypes. PMID: 19966007
- Analysis of the FNR regulon and its interactions with nitrate, nitrite, NarXL, and NarQP. PMID: 16377617
- Studies indicate that only a small fraction of NarX and NarQ remains phosphorylated in the absence of nitrate, leading to efficient dephosphorylation of response regulators by the remaining unphosphorylated sensors. PMID: 18375557
KEGG: ecj:JW1213
STRING: 316385.ECDH10B_1281
