Regulation of the Emissions of the Greenhouse Gas Nitrous Oxide by the Soybean Endosymbiont Bradyrhizobium diazoefficiens

Abstract

The greenhouse gas nitrous oxide (N2O) has strong potential to drive climate change. Soils are a major source of N2O, with microbial nitrification and denitrification being the primary processes involved in such emissions. The soybean endosymbiont Bradyrhizobium diazoefficiens is a model microorganism to study denitrification, a process that depends on a set of reductases, encoded by the napEDABC, nirK, norCBQD, and nosRZDYFLX genes, which sequentially reduce nitrate (NO3−) to nitrite (NO2−), nitric oxide (NO), N2O, and dinitrogen (N2). In this bacterium, the regulatory network and environmental cues governing the expression of denitrification genes rely on the FixK2 and NnrR transcriptional regulators. To understand the role of FixK2 and NnrR proteins in N2O turnover, we monitored real-time kinetics of NO3−, NO2−, NO, N2O, N2, and oxygen (O2) in a fixK2 and nnrR mutant using a robotized incubation system. We confirmed that FixK2 and NnrR are regulatory determinants essential for NO3− respiration and N2O reduction. Furthermore, we demonstrated that N2O reduction by B. diazoefficiens is independent of canonical inducers of denitrification, such as the nitrogen oxide NO3−, 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 N2O turnover in B. diazoefficiens.

Nitric Oxide, Nitric Oxide Formers and Their Physiological Impacts in Bacteria

“…Expression of the hao gene may not be subjected to O 2 repression, as NO could be generated by HAO under anerobic or aerobic conditions [36], although evidence is not available at present. In addition to O 2 , the key NOx in denitrification and dissimilatory NO 3 − reduction, and environmental factors such as pH, and various metals (Mo, Fe, or Cu) that function as cofactors, have all been shown to influence expression of genes in these pathways [143][144][145][146][147]. The major regulators mediating responses to O 2 and NOx belong to the cyclic AMP receptor protein (CRP)/fumarate and nitrate reductase regulation (FNR) superfamily, all referring to the same type of regulatory protein with similar domain structures.…”

Molecular Advances in Microbial Metabolism

“…Parejo and co-workers describe a fine-tuning modulation of oxidation-mediated posttranslational control of the FixK 2 factor in vivo [6]. Bueno and coworkers reported that FixK 2 and NnrR are regulatory determinants essential for denitrification in this bacterium [7]. Furthermore, the authors demonstrate that N 2 O reduction by B. diazoefficiens is independent of canonical inducers of denitrification, as the nitrogen oxide NO 3 − , and it is negatively affected by extreme values of pH (both acidic and alkaline conditions).…”