AUTHOR=Torres María J. , Bueno Emilio , Jiménez-Leiva Andrea , Cabrera Juan J. , Bedmar Eulogio J. , Mesa Socorro , Delgado María J. 

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

JOURNAL=Frontiers in Microbiology

VOLUME=8

YEAR=2017

URL=https://www.frontiersin.org/journals/microbiology/articles/10.3389/fmicb.2017.01621

DOI=10.3389/fmicb.2017.01621

ISSN=1664-302X

ABSTRACT=<p>The powerful greenhouse gas, nitrous oxide (N<sub>2</sub>O) has a strong potential to drive climate change. Soils are the major source of N<sub>2</sub>O and microbial nitrification and denitrification the main processes involved. The soybean endosymbiont <italic>Bradyrhizobium diazoefficiens</italic> is considered a model to study rhizobial denitrification, which depends on the <italic>napEDABC, nirK, norCBQD</italic>, and <italic>nosRZDYFLX</italic> genes. In this bacterium, the role of the regulatory cascade FixLJ-FixK<sub>2</sub>-NnrR in the expression of <italic>napEDABC, nirK</italic>, and <italic>norCBQD</italic> genes involved in N<sub>2</sub>O synthesis has been previously unraveled. However, much remains to be discovered regarding the regulation of the respiratory N<sub>2</sub>O reductase (N<sub>2</sub>OR), the key enzyme that mitigates N<sub>2</sub>O emissions. In this work, we have demonstrated that <italic>nosRZDYFLX</italic> genes constitute an operon which is transcribed from a major promoter located upstream of the <italic>nosR</italic> gene. Low oxygen was shown to be the main inducer of expression of <italic>nosRZDYFLX</italic> genes and N<sub>2</sub>OR activity, FixK<sub>2</sub> being the regulatory protein involved in such control. Further, by using an <italic>in vitro</italic> transcription assay with purified FixK<sub>2</sub> protein and <italic>B. diazoefficiens</italic> RNA polymerase we were able to show that the <italic>nosRZDYFLX</italic> genes are direct targets of FixK<sub>2</sub>.</p>