NITROGEN AND OXYGEN REGULATION OF BACILLUS-SUBTILIS NASDEF ENCODING NADH-DEPENDENT NITRITE REDUCTASE BY TNRA AND RESDE

The nitrate and nitrite reductases of Bacillus subtilis have two diffe rent physiological functions. Under conditions of nitrogen limitation, these enzymes catalyze the reduction of nitrate via nitrite to ammoni a for the anabolic incorporation of nitrogen into biomolecules. They a lso function catabolically in anaerobic respiration, which involves th e use of nitrate and nitrite as terminal electron accepters. Two disti nct nitrate reductases, encoded by narGHI and nasBC, function in anabo lic and catabolic nitrogen metabolism, respectively. However, as repor ted herein, a single NADH-dependent, soluble nitrite reductase encoded by the nasDE genes is required for both catabolic and anabolic proces ses. The nasDE genes, together with nasBC (encoding assimilatory nitra te reductase) and nasF (required for nitrite reductase siroheme cofact or formation), constitute the nas operon, Data presented show that tra nscription of nasDEF is driven not only by the previously characterize d nas operon promoter but also hom an internal promoter residing betwe en the nasC and nasD genes. Transcription from both promoters is activ ated by nitrogen limitation during aerobic growth by the nitrogen regu lator, TnrA However, under conditions of oxygen limitation, nasDEF exp ression and nitrite reductase activity were significantly induced. Ana erobic induction of nasDEF required the ResDE two-component regulatory system and the presence of nitrite, indicating partial coregulation o f NasDEF with the respiratory nitrate reductase NarGHI during nitrate respiration.