Regulation of the Emissions of the Greenhouse Gas Nitrous Oxide by the Soybean Endosymbiont .

The greenhouse gas nitrous oxide (NO) has strong potential to drive climate change. Soils are a major source of NO, with microbial nitrification and denitrification being the primary processes involved in such emissions. The soybean endosymbiont is a model microorganism to study denitrification, a process that depends on a set of reductases, encoded by the , , , and genes, which sequentially reduce nitrate (NO) to nitrite (NO), nitric oxide (NO), NO, and dinitrogen (N). In this bacterium, the regulatory network and environmental cues governing the expression of denitrification genes rely on the FixK and NnrR transcriptional regulators. To understand the role of FixK and NnrR proteins in NO turnover, we monitored real-time kinetics of NO, NO, NO, NO, N, and oxygen (O) in a and mutant using a robotized incubation system. We confirmed that FixK and NnrR are regulatory determinants essential for NO respiration and NO reduction. Furthermore, we demonstrated that NO reduction by is independent of canonical inducers of denitrification, such as the nitrogen oxide NO, and it is negatively affected by acidic and alkaline conditions. These findings advance the understanding of how specific environmental conditions and two single regulators modulate NO turnover in .