Either of two functionally redundant sensor proteins, NarX and NarQ, is sufficient for nitrate regulation in Escherichia coli K-12.
Nitrate acts through the response regulator NarL to activate and repress anaerobic respiratory gene expression in Escherichia coli. The narX gene product encodes a cognate sensor (histidine protein kinase). However, previous work discovered that NarL-mediated nitrate regulation is essentially normal in delta narX deletion mutants. In other two-component regulatory systems studied, the cognate sensor gene is essential for normal regulation. We suggested that NarX-mediated signal transduction reactions are also provided by a functionally redundant nitrate sensor, NarQ. We report here the identification and analysis of narQ insertion mutants. In narX+ strains, a narQ::Tn10 insertion had no perceptible effect on nitrate regulation. However, the same narQ::Tn10 insertion eliminated nitrate regulation when present in delta narX deletion strains. Thus, either narX+ or narQ+ was sufficient for essentially normal NarL-mediated nitrate regulation. The narQ gene mapped to 53 minutes on the E. coli genetic map, a location distinct from all known nitrate regulatory or target genes. The predicted NarQ sequence shares substantial similarity with NarX, particularly in the histidine protein kinase region and in a region of shared similarity with the methyl-accepting chemotaxis proteins. Both NarQ and NarX apparently have N-terminal periplasmic domains, but the primary structures of these regions are largely dissimilar in the two sequences. Analysis of narX* and narL missense alleles in narQ+ versus narQ::Tn10 backgrounds suggests that NarQ and NarX may have subtle functional differences.
Rabin
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Section of Microbiology, Cornell University, Ithaca, NY 14853-8101.
Stewart
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D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D002874:Chromosome Mapping; D003001:Cloning, Molecular; D004268:DNA-Binding Proteins; D004926:Escherichia coli / Q000378:metabolism*; D029968:Escherichia coli Proteins; D015964:Gene Expression Regulation, Bacterial; D005798:Genes, Bacterial; D005809:Genes, Regulator; D006639:Histidine / Q000737:chemistry; D008969:Molecular Sequence Data; D009566:Nitrates / Q000378:metabolism*; D016415:Sequence Alignment; D015398:Signal Transduction; D013329:Structure-Activity Relationship
D001426:Bacterial Proteins; D004268:DNA-Binding Proteins; D029968:Escherichia coli Proteins; D009566:Nitrates; C069780:NarL protein, E coli; D006639:Histidine