title: An integrated biochemical system for nitrate assimilation and nitric oxide detoxification in Bradyrhizobium japonicum.
abstract: 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.
authors.affiliation: Estación Experimental del Zaidín, CSIC, PO Box 419, Granada 18080, Spain.
authors.forename: Juan J
authors.initials: JJ
authors.lastname: Cabrera
authors.affiliation: Estación Experimental del Zaidín, CSIC, PO Box 419, Granada 18080, Spain.
authors.forename: Ana
authors.initials: A
authors.lastname: Salas
authors.affiliation: Estación Experimental del Zaidín, CSIC, PO Box 419, Granada 18080, Spain.
authors.forename: María J
authors.initials: MJ
authors.lastname: Torres
authors.affiliation: Estación Experimental del Zaidín, CSIC, PO Box 419, Granada 18080, Spain.
authors.forename: Eulogio J
authors.initials: EJ
authors.lastname: Bedmar
authors.affiliation: Centre for Molecular and Structural Biochemistry, University of East Anglia, Norwich Research Park, Norwich NR4 7TJ, U.K. School of Biological Sciences, University of East Anglia, Norwich Research Park, Norwich NR4 7TJ, U.K.
authors.forename: David J
authors.initials: DJ
authors.lastname: Richardson
authors.affiliation: Centre for Molecular and Structural Biochemistry, University of East Anglia, Norwich Research Park, Norwich NR4 7TJ, U.K. School of Biological Sciences, University of East Anglia, Norwich Research Park, Norwich NR4 7TJ, U.K. a.gates@uea.ac.uk mariajesus.delgado@eez.csic.es.
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Gates
authors.affiliation: Estación Experimental del Zaidín, CSIC, PO Box 419, Granada 18080, Spain a.gates@uea.ac.uk mariajesus.delgado@eez.csic.es.
authors.forename: María J
authors.initials: MJ
authors.lastname: Delgado
mesh_terms: D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D020369:Bradyrhizobium / Q000201:enzymology / Q000235:genetics / Q000378:metabolism*; D015964:Gene Expression Regulation, Bacterial; D026901:Membrane Transport Proteins / Q000378:metabolism; D009566:Nitrates / Q000378:metabolism*; D009569:Nitric Oxide / Q000378:metabolism*; D009572:Nitrite Reductases / Q000235:genetics / Q000378:metabolism; D009573:Nitrites / Q000378:metabolism
chemical_list: D001426:Bacterial Proteins; D026901:Membrane Transport Proteins; D009566:Nitrates; D009573:Nitrites; D009569:Nitric Oxide; D009572:Nitrite Reductases
keywords: bacterial denitrification; bacterial haemoglobin; nitrate reduction; nitric oxide reductase; nitrite reduction