Expression of the Escherichia coli NRZ nitrate reductase is highly growth phase dependent and is controlled by RpoS, the alternative vegetative sigmafactor

In the absence of oxygen, many bacteria preferentially use nitrate as a ter minal electron acceptor for anaerobic respiration. In Escherichia coli, the re are two membrane-bound, differentially regulated nitrate reductases. Whi le the physiological basis for this metabolic redundancy is not completely understood, during exponential growth, synthesis of NRA is greatly induced by anaerobiosis plus nitrate, whereas NRZ is expressed at a low level that is not influenced by anaerobiosis or nitrate. In the course of identifying genes controlled by the stationary phase regulatory factor RpoS (sigma(s)), we found that the expression of NRZ is induced during entry into stationar y phase and highly dependent on this alternative sigma factor. Expression s tudies, using operon fusions and nitrate reductase assays, revealed that th e NRZ operon is controlled mainly at the level of transcription and is indu ced 10-fold at the onset of stationary phase in rich media. Consistent with previous reports of RpoS expression, the RpoS dependency of NRZ in minimal media was very high (several hundredfold). We also observed a fivefold sta tionary phase induction of NRZ in an rpoS background, indicating that other regulatory factors, besides RpoS, are probably involved in transcriptional control of NRZ. The RpoS dependence of NRZ expression was confirmed by Nor thern analyses using RNA extracted from wild-type and rpoS(-) strains sampl ed in exponential and stationary phase. In toto, these data indicate that R poS-mediated regulation of NRZ may be an important physiological adaptation that allows the cell to use nitrate under stress-associated conditions.