Co-regulation of nitrate assimilation and nitric oxide metabolism in Bradyrhizobium japonicum

ANA SALAS; JUAN J. CABRERA; M. F. LÓPEZ; MARÍA J TORRES; SILVINA L. LOPEZ-GARCIA; EULOGIO J BEDMAR; DAVID J. RICHARDSON; ANDREW J. GATES; MARÍA J. DELGADO

Vimeiro

Encuentro; Bacterial Electron Transfer Processes and their Regulation; 2015

European Federation of Biotechnology, Microbial Physiology Section

Bradyrhizobium japonicum is a soil bacterium that establishes nitrogen-fixing symbiotic associations with soybean plants, and may denitrify under free living and symbiotic conditions. This nitrate-utilising bacterium also expresses an assimilatory nitrate/nitrite reductase system. In contrast to other bacteria that assimilate nitrate, the B. japonicum genes encoding the nitrate reductase (nasC) and nitrite reductase (nirA) are located at distinct loci on the chromosome. nasC is located in a cluster containing genes that encode a nitrate/nitrite transporter (NasA), a flavoprotein (Flp) and a single domain haemoglobin (Bjgb) that functions to mitigate cytotoxic nitric oxide formed as a by-product of nitrate assimilation under anoxic free-living conditions. However, nirA clusters with genes for a putative nitrate/nitrite responsive regulator system (NasTS). Here we have demonstrated that nasA, bjgb, flp and nasC comprise a transcriptional unit. -galactosidase activity of a transcriptional nasA-lacZ promoter fusion showed that expression of these genes is repressed by ammonium or glutamate and induced by nitrate and nitric oxide. This work has revealed the involvement of the regulatory proteins NtrBC, NasTS, and NnrR in the expression of the nasA-bjgb-flp-nasC operon, thus implying a co-regulation of nitrate assimilation and NO-detoxification in B. japonicum.