Dual Oxygen-Responsive Control by RegSR of Nitric Oxide Reduction in the Soybean Endosymbiont <i>Bradyrhizobium diazoefficiens</i>

<b><i>Aims:</i></b> To investigate the role of the RegSR-NifA regulatory cascade in the oxygen control of nitric oxide (NO) reduction in the soybean endosymbiont <i>Bradyrhizobium diazoefficiens</i>. <b><i>Results:</i></b> We have performed an integrated study of <i>norCBQD</i> expression and NO reductase activity in <i>regR</i>, <i>regS</i>₁, <i>regS</i>₂, <i>regS</i>_1/2, and <i>nifA</i> mutants in response to microoxia (2% O₂) or anoxia. An activating role of RegR and NifA was observed under anoxia. In contrast, under microaerobic conditions, RegR acts as a repressor by binding to a RegR box located between the -10 and -35 regions within the <i>norCBQD</i> promoter. In addition, both RegS₁ and RegS₂ sensors cooperated with RegR in repressing <i>norCBQD</i> genes. <b><i>Innovation:</i></b> NO is a reactive gas that, at high levels, acts as a potent inhibitor of symbiotic nitrogen fixation. In this paper, we report new insights into the regulation of NO reductase, the major enzyme involved in NO removal in rhizobia. This knowledge will be crucial for the development of new strategies and management practices in agriculture, in particular, for improving legume production. <b><i>Conclusion:</i></b> Our results demonstrate, for the first time, a dual control of the RegSR two-component regulatory system on <i>norCBQD</i> genes control in response to oxygen levels. <i>Antioxid. Redox Signal.</i> 42, 408-420.