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Reconciling the irreconcilable is a primary struggle in aerobic nitrogen-fixing bacteria. Although nitrogenase is oxygen and reactive oxygen species-labile, oxygen tension is required to sustain respiration. In the nitrogen-fixing  Frankia , various strategies have been developed through evolution to control the respiration and nitrogen-fixation balance. Here, we assessed the effect of different oxygen tensions on  Frankia  sp. strain CcI3 growth, vesicle production, and gene expression under different oxygen tensions. Both biomass and vesicle production were correlated with elevated oxygen levels under both nitrogen-replete and nitrogen-deficient conditions. The mRNA levels for the nitrogenase structural genes ( nif HDK) were high under hypoxic and hyperoxic conditions compared to oxic conditions. The mRNA level for the hopanoid biosynthesis genes ( sqh C and  hpn C) was also elevated under hyperoxic conditions suggesting an increase in the vesicle envelope. Under nitrogen-deficient conditions, the  hup 2 mRNA levels increased with hyperoxic environment, while  hup 1 mRNA levels remained relatively constant. Taken together, these results indicate that  Frankia  protects nitrogenase by the use of multiple mechanisms including the vesicle-hopanoid barrier and increased respiratory protection.

Contrasted Reactivity to Oxygen Tensions inFrankiasp. Strain CcI3 throughout Nitrogen Fixation and Assimilation