Abstract
Members of Rhizobiaceae contain a homologue of the iron-responsive regulatory protein RirA. In different bacteria, RirA acts as a repressor of iron uptake systems under iron-replete conditions and contributes to ameliorate cell damage during oxidative stress. In Rhizobium leguminosarum and Sinorhizobium meliloti, mutations in rirA do not impair symbiotic nitrogen fixation. In this study, a rirA mutant of broad host range S. fredii HH103 has been constructed (SVQ780) and its free-living and symbiotic phenotypes evaluated. No production of siderophores could be detected in either the wild-type or SVQ780. The rirA mutant exhibited a growth advantage under iron-deficient conditions and hypersensitivity to hydrogen peroxide in iron-rich medium. Transcription of rirA in HH103 is subject to autoregulation and inactivation of the gene upregulates fbpA, a gene putatively involved in iron transport. The S. fredii rirA mutant was able to nodulate soybean plants, but symbiotic nitrogen fixation was impaired. Nodules induced by the mutant were poorly infected compared to those induced by the wild-type. Genetic complementation reversed the mutant’s hypersensitivity to H2O2, expression of fbpA, and symbiotic deficiency in soybean plants. This is the first report that demonstrates a role for RirA in the Rhizobium-legume symbiosis.
Highlights
As for almost all living organisms, iron is an essential nutrient for bacteria but in excess it becomes toxic
Reactive oxygen species (ROS) such as H2O2 generated during aerobic metabolism in combination with ferrous iron leads to the production of hydroxyl radicals (OH), which are extremely reactive oxidants capable of damaging DNA, proteins, and lipids [2]
I.e., an increased ROS production that cannot be balanced by antioxidants, toxicity caused by iron is more critical
Summary
As for almost all living organisms, iron is an essential nutrient for bacteria but in excess it becomes toxic This active redox metal forms part of the prosthetic group (heme or iron–sulfur clusters) of many proteins that participate in important metabolic processes such as respiration, central metabolism, redox stress resistance, or nitrogen fixation. Symbiotic nitrogen fixation is highly dependent on iron because different proteins involved in the process require the metal to be functional This is the case of the nitrogenase complex responsible for the nitrogen fixation reaction, or the abundant nodule protein leghemoglobin that buffers oxygen levels to avoid inactivation of nitrogenase activity. In rhizobia distinct from Bradyrhizobiaceae, iron homeostasis is achieved with the participation of a second regulatory protein, RirA (Rhizobial Iron Regulator A). In S. fredii HH103, RirA is important for oxidative stress resistance and for effective symbiosis with soybean plants
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