Nitrate- as well as nitrite-dependent gene expression in Escherichia coli is accomplished by the Nar two-component regulatory system, composed of the NarX, NarQ, NarL, and NarP proteins (4). The Nar phosphorelay system is unusual in that it has two sensor-transmitter members, NarX and NarQ, that can independently detect nitrate or nitrite signals. It also has two response regulators, NarL and NarP, that interact with DNA. The reception of either the nitrate or nitrite signal by NarX or NarQ elicits a message transfer to NarL and NarP in the form of covalent protein phosphorylation (2, 5, 6) that activates the two DNA binding proteins so they can then modulate gene expression.

NarL-phosphate and NarP-phosphate control the transcription of many genes involved in anaerobic respiration and fermentation (4). The respiratory pathway genes include those for two cellular nitrate reductases (narGHJI and napA), two nitrite reductases (nirBDC and nrfABCDEFG), a nitrite exporter (narK), a formate dehydrogenase (fdnGHI), a dimethyl sulfoxide/trimethylamine-N-oxide reductase (dmsABC), and a fumarate reductase (frdABCD). The fermentation pathway genes include those for alcohol dehydrogenase (adhE) and pyruvate formate lyase (pfl). Finally, NarX and NarQ also possess a cophosphatase activity that stimulates the rate of NarL-phosphate and NarP-phosphate dephosphorylation to recycle these DNA binding proteins to their inactive states (2, 5, 6).

NarX and NarQ have two transmembrane regions near the N-terminal end and a region in between exposed to the periplasm. Mutations in the periplasmic region of NarX or NarQ result in mutants that either have lost the ability to respond to nitrate and nitrite or behave as if nitrate or nitrite were present even in their absence (1). From these data it has been concluded that nitrate and nitrite are detected in the periplasmic region, and that the signal is transduced across the cell membrane to the response regulator proteins. Both NarL and NarP have been demonstrated to bind to specific site in target operons (3).

Instead of having 2 Histidine kinase (NarX NarQ) and 2 response regulator (NarL NarP) in E.coi, nitrate-dependent gene expression in Haemophilus ducreyi is mediated by 1 HK (NarQ) and 1 RR (NarP).

Model for nitrate/nitrite control of anaerobic gene expression in Haemophilus ducreyi

NarQ Sequence Alignment

NarQ Sequence Alignment with colour indications

References

1.Cavicchioli, R., R. C. Chiang, L. V. Kalman, and R. P. Gunsalus. 1996. Role of the periplasmic domain of the Escherichia coli NarX sensor-transmitter protein in nitrate-dependent signal transduction and gene regulation. molecular microbiology. 21(5):901-11.
2.Cavicchioli, R., I. Schroder, M. Constanti, and R. Gunsalus. 1995. The NarX and NarQ sensor-transmitter proteins of Escherichia coli each require two conserved histidines for nitrate-dependent signal offnsduction to NarL. J. Bacteriol. 177(9):2416-2424.
3.Darwin, A. J., K. L. Tyson, S. J. Busby, and V. Stewart. 1997. Differential regulation by the homologous response regulators NarL and NarP of Escherichia coli K-12 depends on DNA binding site arrangement. molecular microbiology. 25(3):583-95.
4.Gunsalus, R. P. 1992. Control of electron flow in Escherichia coli: coordinated transcription of respiratory pathway genes. journal of bacteriology. 174(22):7069-74.
5.Schroder, I., C. Wolin, R. Cavicchioli, and R. Gunsalus. 1994. Phosphorylation and dephosphorylation of the NarQ, NarX, and NarL proteins of the nitrate-dependent two-component regulatory system of Escherichia coli. J. Bacteriol. 176(16):4985-4992.
6.Walker, M., and J. DeMoss. 1993. Phosphorylation and dephosphorylation catalyzed in vitro by purified components of the nitrate sensing system, NarX and NarL. J. Biol. Chem. 268(12):8391-8393.

This analysis was prepared by Gary Xie, and Staff. Please direct questions concerning this analysis to Gary Xie.