Nitrate respiration in relation to facultative metabolism in enterobacteria

Abstract

Nitrate is the most efficient electron acceptor for anaerobic respiration in members of the Enterobacteriaceae. The metabolism of nitrate-respiring cells is in an intermediate state between 'aerobiosis' and 'anaerobiosis'; for example, the tricarboxylic acid cycle is incomplete, but other anaerobic respiratory and fermentation enzymes are not synthesized. The nitrate respiratory chain functions with electron donors such as formate, reduced nicotinamide adenine dinucleotide, and lactate and requires quinone and specific cytochromes b. The terminal enzyme, nitrate reductase, is a membrane-bound complex of at least three subunits. The active-site subunit (α) contains molybdenum cofactor and nonheme iron and faces the cytoplasm. The cytochrome b subunit (γ) faces the periplasm. Thus, it is hypothesized that a chemiosmotic gradient is formed by the release of two protons in the periplasm (concomitant with quinol oxidation) coupled to the consumption of two protons in the cytoplasm (concomitant with nitrate reduction). Chlorate-resistant mutants (chl) lack the activities of all molybdo-enzymes and are deficient in molybdenum cofactor synthesis or processing. The structural genes for nitrate reductase are organized in the nar operon. Nitrate reductase synthesis is induced by anaerobiosis, and anaerobically it is further induced by nitrate. Anaerobic induction requires the fnr gene product, which activates transcription of genes for anaerobic respiratory enzymes, but not of genes for other anaerobic enzymes. Nitrate induction requires the narL gene product, which activates transcription of the nar operon, and may repress transcription of genes for other anaerobic respiratory enzymes.