Nitrous oxide (N2O) is an important greenhouse gas (GHG) with substantial global warming potential and also contributes to ozone depletion through photochemical nitric oxide (NO) production in the stratosphere. The negative effects of N2O on climate and stratospheric ozone make N2O mitigation an international challenge. More than 60% of global N2O emissions are emitted from agricultural soils mainly due to the application of synthetic nitrogen-containing fertilizers. Thus, mitigation strategies must be developed which increase (or at least do not negatively impact) on agricultural efficiency whilst decrease the levels of N2O released. This aim is particularly important in the context of the ever expanding population and subsequent increased burden on the food chain. More than two-thirds of N2O emissions from soils can be attributed to bacterial and fungal denitrification and nitrification processes. In ammonia-oxidizing bacteria, N2O is formed through the oxidation of hydroxylamine to nitrite. In denitrifiers, nitrate is reduced to N2 via nitrite, NO and N2O production. In addition to denitrification, respiratory nitrate ammonification (also termed dissimilatory nitrate reduction to ammonium) is another important nitrate-reducing mechanism in soil, responsible for the loss of nitrate and production of N2O from reduction of NO that is formed as a by-product of the reduction process. This review will synthesize our current understanding of the environmental, regulatory and biochemical control of N2O emissions by nitrate-reducing bacteria and point to new solutions for agricultural GHG mitigation.

Manuscript number NOX_2016_125Title Disparate response to microoxia and nitrogen oxides of the Bradyrhizobium japonicum napEDABC, nirK, and norCBQD denitrification genes Article type Research Paper AbstractExpression of the Bradyrhizobium japonicum napEDABC, nirK and norCBQD denitrification genes requires low oxygen (O2) tension and nitrate (NO3-), through a regulatory network comprised of two coordinated cascades, FixLJ-FixK2-NnrR and RegSR/NifA. To precisely understand how these signals are integrated in the FixLJ-FixK2-NnrR circuit, we analyzed β-Galactosidase activities from napE-lacZ, nirK-lacZ and norC-lacZ fusions and performed analyses of NapC and NorC levels as well as periplasmic nitrate reductase (Nap) activity, in B. japonicum wildtype and fixK2 and nnrR mutant backgrounds. While microoxic conditions (2% O2 at headspace) were sufficient to induce expression of napEDABC and nirK genes and this control depends on FixK2, norCBQD expression requires, in addition to microoxia, nitric oxide gas (NO) and both FixK2 and NnrR transcription factors. Purified FixK2 protein directly interacted and activated transcription in collaboration with B. japonicum RNA polymerase from the napEDABC and nirK promoters, but not from the norCBQD promoter. Further, recombinant NnrR protein bound exclusively to the norCBQD promoter in an oxygen-sensitive manner. Our work suggest a disparate regulation of B. japonicum denitrifying genes expression with regard to their dependency to microoxia, nitrogen oxides (NOx), and the regulatory proteins FixK2 and NnrR. In this control, expression of napEDABC and nirK genes requires microoxic conditions and directly depends on FixK2, while expression of norCBQD genes relies on NO, being NnrR the candidate which directly interacts with the norCBQD promoter. Bueno_et_al_Table_1_20161106.docx [Table] Bueno_et_al_Supplementary data_20161106.pdf [Supporting File] To view all the submission files, including those not included in the PDF, click on the manuscript title on your EVISE Homepage, then click 'Download zip file'. Keywords

We report a dual functional system for bacterial nitrate (NO3−) assimilation and nitric oxide (NO) detoxification. The assimilatory NO3− reductase (NasC) can generate nitric oxide (NO). Co-expression of an NO-detoxification system acts to counteract accumulation of cytotoxic NO during anaerobic NO3−-dependent growth.

The powerful greenhouse gas, nitrous oxide (N2O) has a strong potential to drive climate change. Soils are the major source of N2O and microbial nitrification and denitrification the main processes involved. The soybean endosymbiont Bradyrhizobium diazoefficiens is considered a model to study rhizobial denitrification, which depends on the napEDABC, nirK, norCBQD, and nosRZDYFLX genes. In this bacterium, the role of the regulatory cascade FixLJ-FixK2-NnrR in the expression of napEDABC, nirK, and norCBQD genes involved in N2O synthesis has been previously unraveled. However, much remains to be discovered regarding the regulation of the respiratory N2O reductase (N2OR), the key enzyme that mitigates N2O emissions. In this work, we have demonstrated that nosRZDYFLX genes constitute an operon which is transcribed from a major promoter located upstream of the nosR gene. Low oxygen was shown to be the main inducer of expression of nosRZDYFLX genes and N2OR activity, FixK2 being the regulatory protein involved in such control. Further, by using an in vitro transcription assay with purified FixK2 protein and B. diazoefficiens RNA polymerase we were able to show that the nosRZDYFLX genes are direct targets of FixK2.