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Klebsiella oxytoca can assimilate nitrate and nitrite by using enzymes encoded by thenasFEDCBA operon. Expression of thenasF operon is controlled by general nitrogen regulation (Ntr) via the NtrC transcription activator and by pathway-specific nitrate and nitrite induction via the NasR transcription antiterminator. This paper reports our analysis ofnasR gene expression. We constructed strains bearing single-copy Φ(nasR-lacZ ) operon fusions within the chromosomalrhaBAD-rhaSR locus. The expression of ΔrhaBS ::[Φ(nasR-lacZ )] operon fusions was induced about 10-fold during nitrogen-limited growth. Induction was reduced in bothntrC andrpoN null mutants, indicating that Ntr control ofnasR gene expression requires the NtrC and ςN (ς54 ) proteins. Sequence inspection of thenasR control region reveals an apparent ςN -dependent promoter but no apparent NtrC protein binding sites. Analysis of site-specific mutations coupled with primer extension analysis authenticated the ςN -dependentnasR promoter. Fusion constructs with only about 70 nucleotides (nt) upstream of the transcription initiation site exhibited patterns of β-galactosidase expression indistinguishable from Φ(nasR-lacZ ) constructs with about 470 nt upstream. Expression was independent of the Nac protein, implying that NtrC is a direct activator ofnasR transcription. Together, these results indicate thatnasR gene expression does not require specific upstream NtrC-binding sequences, as previously noted forargT gene expression inSalmonella typhimurium (G. Schmitz, K. Nikaido, and G. F.-L. Ames, Mol. Gen. Genet. 215:107–117, 1988).

General Nitrogen Regulation of Nitrate Assimilation Regulatory Gene nasR Expression in Klebsiella oxytoca M5al