title: 'Locked-on' and 'locked-off' signal transduction mutations in the periplasmic domain of the Escherichia coli NarQ and NarX sensors affect nitrate- and nitrite-dependent regulation by NarL and NarP.
abstract: The Escherichia coli NarX, NarQ, NarL and NarP proteins comprise a two-component regulatory system that controls the expression of many anaerobic electron-transport and fermentation-related genes in response to nitrate and nitrite. Either of the two sensor-transmitter proteins, NarX and NarQ, can activate the response-regulator proteins, NarL and NarP, which in turn are able to bind at their respective DNA regulatory sites to modulate gene expression. NarX contains a conserved 17 amino acid sequence, designated the 'P-box' element, that is essential for nitrate sensing. In this study we characterize narQ mutants that also confer altered nitrate control of NarL-dependent nitrate reductase (narGHJI) and fumarate reductase (frdABCD) gene expression. While some narQ mutations cause the constitutive activation or repression of reporter-gene expression even when the cells are grown in the absence of the nitrate signal (i.e. a 'locked-on' phenotype), other mutations abolish nitrate-dependent control (i.e. a 'locked-off' phenotype). Interestingly the narQ (A42-->T) and narQ (R50-->Q) mutations along with the analogous narX18 (A46-->T) and narX902 (R54-->E) mutations also confer a 'locked-on' or a 'locked-off' phenotype in response to nitrite, the second environmental signal detected by NarQ and NarX. Furthermore, these narQ and narX mutations also affect NarP-dependent gene regulation of nitrite reductase (nrfABCDEFG) and aeg-46.5 gene expression in response to nitrite. We therefore propose that the NarQ sensor-transmitter protein also detects nitrate and nitrite in the periplasmic space via its periplasmic domain. A signal transduction model, which we previously proposed for NarX, is now extended to NarQ, in which a nitrate- or nitrite-detection event in the periplasmic region of the cell is followed by a signal transduction event through the inner membrane to the cytoplasmic domain of NarQ and NarX proteins to modulate their protein kinase/phosphatase activities.
authors.affiliation: Department of Microbiology and Molecular Genetics, and the Molecular Biology Institute, University of California, Los Angeles 90095-1489, USA.
authors.forename: R C
authors.initials: RC
authors.lastname: Chiang
authors.forename: R
authors.initials: R
authors.lastname: Cavicchioli
authors.forename: R P
authors.initials: RP
authors.lastname: Gunsalus
mesh_terms: D000409:Alanine; D000595:Amino Acid Sequence; D001120:Arginine; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D001665:Binding Sites; D003593:Cytoplasm; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D004926:Escherichia coli / Q000235:genetics* / Q000378:metabolism; D029968:Escherichia coli Proteins; D015964:Gene Expression Regulation, Bacterial; D007763:Lac Operon; D008565:Membrane Proteins / Q000235:genetics*; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009419:Nerve Tissue Proteins / Q000235:genetics; D050901:Nitrate Reductase; D009565:Nitrate Reductases / Q000235:genetics; D009566:Nitrates / Q000378:metabolism; D009573:Nitrites / Q000378:metabolism; D010750:Phosphoproteins / Q000235:genetics*; D011494:Protein Kinases; D015398:Signal Transduction
chemical_list: D001426:Bacterial Proteins; D004268:DNA-Binding Proteins; D029968:Escherichia coli Proteins; D008565:Membrane Proteins; D009419:Nerve Tissue Proteins; D009566:Nitrates; D009573:Nitrites; D010750:Phosphoproteins; C081466:narP protein, E coli; C088708:narQ protein, E coli; C069780:NarL protein, E coli; D001120:Arginine; D009565:Nitrate Reductases; D050901:Nitrate Reductase; D011494:Protein Kinases; C074787:narX protein, E coli; D000409:Alanine