Disparate response to microoxia and nitrogen oxides of the Bradyrhizobium japonicum napEDABC, nirK and norCBQD denitrification genes

Abstract

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

A common mechanism for efficient N2O reduction in diverse isolates of nodule‐forming bradyrhizobia

“…Previous work by other groups showed that transcription of denitrification genes in the model organism Bradyrhizobium diazoefficiens USDA 110 (earlier called B. japonicum) is regulated by the O 2 sensitive FixJL, the NO 3 − -sensitive NasTS, the NO sensitive NnrR, the redox sensitive RegSR and the RegR and self-induced NifA (Torres et al, 2014;Cabrera et al, 2016). Noticeably, the expression of the nap and nir operons was apparently initiated by microoxic conditions, while that of the nor operon required a nitrogen oxide, probably NO (Bueno et al, 2017). Regulation of nosZ was not investigated, but another study (Torres et al, 2017) suggested microaerobic conditions as the main inducer of nosZ.…”

Diversifying Anaerobic Respiration Strategies to Compete in the Rhizosphere

“…Nar is composed of 3 subunits encoded by narGHI, as found in E. coli, P. fluorescens, P. stutzeri and B. subtilis (Philippot and Højberg, 1999;Lalucat et al, 2006), whereas Nap is composed of 2 subunits encoded by napAB, as found in Rhodabacter sphaeroides and B. japonicum (Reyes et al, 1998;Bedmar et al, 2005). The nar operon is induced under low oxygen partial pressure and in the presence of nitrogen oxides (Philippot et al, 2001;Giannopoulos et al, 2017), whereas the expression of the nap operon can be affected or not by anaerobiosis depending on the organism (Philippot, 2002;Bueno et al, 2017).…”

FixK2 Is the Main Transcriptional Activator of Bradyrhizobium diazoefficiens nosRZDYFLX Genes in Response to Low Oxygen

“…Expression of B. diazoefficiens denitrification genes required low oxygen tension and in the case of norCBQD genes the presence of NO is also needed ( Bueno et al, 2017 ). In this bacterium, perception and transduction of the ‘low-oxygen’ signal are mediated by a complex network comprising two interconnected regulatory cascades, the FixLJ–FixK 2 –NnrR and the RegSR–NifA ( Sciotti et al, 2003 ).…”