An integrated biochemical system for nitrate assimilation and nitric oxide detoxification in Bradyrhizobium japonicum

Rhizobia are recognized to establish N2-fixing symbiotic interactions with legume plants. Bradyrhizobium japonicum, the symbiont of soybeans, can denitrify and grow under free-living conditions with nitrate (NO3−) or nitrite (NO2−) as sole nitrogen source. Unlike related bacteria that assimilate NO3−, genes encoding the assimilatory NO3− reductase (nasC) and NO2− reductase (nirA) in B. japonicum are located at distinct chromosomal loci. The nasC gene is located with genes encoding an ABC-type NO3− transporter, a major facilitator family NO3−/NO2− transporter (NarK), flavoprotein (Flp) and single-domain haemoglobin (termed Bjgb). However, nirA clusters with genes for a NO3−/NO2−-responsive regulator (NasS-NasT). In the present study, we demonstrate NasC and NirA are both key for NO3− assimilation and that growth with NO3−, but not NO2− requires flp, implying Flp may function as electron donor to NasC. In addition, bjgb and flp encode a nitric oxide (NO) detoxification system that functions to mitigate cytotoxic NO formed as a by-product of NO3− assimilation. Additional experiments reveal NasT is required for NO3−-responsive expression of the narK-bjgb-flp-nasC transcriptional unit and the nirA gene and that NasS is also involved in the regulatory control of this novel bipartite assimilatory NO3−/NO2− reductase pathway.