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Expanding the Regulon of the Bradyrhizobium diazoefficiens NnrR Transcription Factor: New Insights Into the Denitrification Pathway
Department of Soil Microbiology and Symbiotic Systems, Estación Experimental del Zaidín, Consejo Superior de Investigaciones Científicas, Granada, Spain.
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2019 (English)In: Frontiers in Microbiology, ISSN 1664-302X, E-ISSN 1664-302X, Vol. 10, article id 1926Article in journal (Refereed) Published
Abstract [en]

Denitrification in the soybean endosymbiont Bradyrhizobium diazoefficiens is controlled by a complex regulatory network composed of two hierarchical cascades, FixLJ-FixK(2)-NnrR and RegSR-NifA. In the former cascade, the CRP/FNR-type transcription factors FixK(2) and NnrR exert disparate control on expression of core denitrifying systems encoded by napEDABC, nirK, norCBQD, and nosRZDFYLX genes in response to microoxia and nitrogen oxides, respectively. To identify additional genes controlled by NnrR and involved in the denitrification process in B. diazoefficiens, we compared the transcriptional profile of an nnrR mutant with that of the wild type, both grown under anoxic denitrifying conditions. This approach revealed more than 170 genes were simultaneously induced in the wild type and under the positive control of NnrR. Among them, we found the cycA gene which codes for the c(550) soluble cytochrome (CycA), previously identified as an intermediate electron donor between the bc(1) complex and the denitrifying nitrite reductase NirK. Here, we demonstrated that CycA is also required for nitrous oxide reductase activity. However, mutation in cycA neither affected nosZ gene expression nor NosZ protein steady-state levels. Furthermore, cycA, nnrR and its proximal divergently oriented nnrS gene, are direct targets for FixK(2) as determined by in vitro transcription activation assays. The dependence of cycA expression on FixK(2) and NnrR in anoxic denitrifying conditions was validated at transcriptional level, determined by quantitative reverse transcription PCR, and at the level of protein by performing heme c-staining of soluble cytochromes. Thus, this study expands the regulon of NnrR and demonstrates the role of CycA in the activity of the nitrous oxide reductase, the key enzyme for nitrous oxide mitigation.

Place, publisher, year, edition, pages
Frontiers Media S.A., 2019. Vol. 10, article id 1926
Keywords [en]
CRP/FNR proteins, in vitro transcription, microoxia, nitrogen oxides, Rhizobia, transcriptomics
National Category
Microbiology
Identifiers
URN: urn:nbn:se:umu:diva-162844DOI: 10.3389/fmicb.2019.01926ISI: 000481765700001PubMedID: 31481951OAI: oai:DiVA.org:umu-162844DiVA, id: diva2:1349661
Available from: 2019-09-09 Created: 2019-09-09 Last updated: 2019-09-09Bibliographically approved

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Bueno, Emilio

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