Denitrification is an alternative form of respiration in which bacteria reduce sequentially nitrate (NO3") or nitrite (NO2") to nitrogen gas (N2) anaerobically. Among the enzymes involved in denitrification, nitrate reductase reduces nitrate to nitrite, and nitrite reductase catalyzes the reduction of nitrite to nitric oxide (NO); nitric oxide is further reduced to nitrous oxide (N2O) by nitric oxide reductase; finally, N2O is converted to N2 by the nitrous oxide reductase enzyme. Reduction of nitrogen oxides is coupled to energy conservation and permits cell growth under oxygen-limiting conditions. Although the denitrification process is initiated by respiratory nitrate reduction, this reaction is not unique to denitrification since it also occurs in ammonification and assimilatory nitrate reduction. Thus it is considered that the defining reaction in denitrification is the reduction of nitrite to the first gaseous intermediate, NO. Comprehensive reviews covering the physiology, biochemistry and molecular genetics of denitrification have been published elsewhere (Zumft 1997; Baker et al. 1998; Watmough et al. 1999; Hendriks et al. 2000). Bradyrhizobium japonicum is a gram-negative soil bacterium with the unique ability to establish an N2-fixing symbiosis with soybeans (Glycine max L. Merr.). B. japonicum cells have been shown to couple nitrate reduction to ATP generation (Bandhari, Nicholas 1984) and to assimilate and denitrify 15N03" simultaneously to 15NH4+ and 15N2, respectively (Vairinhos et al. 1989). The presence of a complete denitrification system has also been demonstrated in bacteroids of B. japonicum', within the nodules, denitrifying activity has been shown to generate ATP and to maintain nodule integrity and nitrogenase activity (O'Hara, Daniel 1985).

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