Periplasmic Nitrate Reductase (NapABC Enzyme) Supports Anaerobic Respiration byEscherichia coliK-12

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Abstract
Periplasmic nitrate reductase (NapABC enzyme) has been characterized from a variety of proteobacteria, especiallyParacoccus pantotrophus. Whole-genome sequencing ofEscherichia colirevealed the structural genesnapFDAGHBC, which encode NapABC enzyme and associated electron transfer components.E. colialso expresses two membrane-bound proton-translocating nitrate reductases, encoded by thenarGHJIandnarZYWVoperons. We measured reduced viologen-dependent nitrate reductase activity in a series of strains with combinations ofnarandnapnull alleles. ThenapFoperon-encoded nitrate reductase activity was not sensitive to azide, as shown previously for theP. pantotrophusNapA enzyme. A strain carrying null alleles ofnarGandnarZgrew exponentially on glycerol with nitrate as the respiratory oxidant (anaerobic respiration), whereas a strain also carrying a null allele ofnapAdid not. By contrast, the presence ofnapA+had no influence on the more rapid growth ofnarG+strains. These results indicate that periplasmic nitrate reductase, like fumarate reductase, can function in anaerobic respiration but does not constitute a site for generating proton motive force. The time course of Φ(napF-lacZ) expression during growth in batch culture displayed a complex pattern in response to the dynamic nitrate/nitrite ratio. Our results are consistent with the observation that Φ(napF-lacZ) is expressed preferentially at relatively low nitrate concentrations in continuous cultures (H. Wang, C.-P. Tseng, and R. P. Gunsalus, J. Bacteriol. 181:5303-5308, 1999). This finding and other considerations support the hypothesis that NapABC enzyme may function inE. coliwhen low nitrate concentrations limit the bioenergetic efficiency of nitrate respiration via NarGHI enzyme.

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