1887

Abstract

In , nitrogen fixation is regulated in response to oxygen concentration through the FixL-FixJ two-component system (TCS). Besides this conserved TCS, the field isolate SM11 also encodes the hFixL-FxkR TCS, which is responsible for the microoxic response in . Through genetic and physiological assays, we evaluated the role of the hFixL-FxkR TCS in SM11. Our results revealed that this regulatory system activates the expression of a f orthologue (a), in response to low oxygen concentration. Null mutations in either hFixL or FxkR promote upregulation of 1, a direct target of FixJ. Furthermore, the absence of this TCS translates into higher nitrogen fixation values as well as higher expression of 1 in nodules. Individual mutations in each of the -like regulators encoded in the SM11 genome do not completely restrict 1 or 2 expression, pointing towards redundancy among these regulators. Both copies of are necessary to achieve optimal levels of nitrogen fixation. This work provides evidence that the hFixL-FxkR TCS is activated in response to low oxygen concentration in SM11 and that it negatively regulates the expression of 1, 1 and nitrogen fixation.

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2016-06-01
2019-10-14
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