| Summary: | Members of <i>Rhizobiaceae</i> 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 <i>Rhizobium leguminosarum</i> and <i>Sinorhizobium meliloti</i>, mutations in <i>rirA</i> do not impair symbiotic nitrogen fixation. In this study, a <i>rirA</i> mutant of broad host range <i>S. fredii</i> 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 <i>rirA</i> mutant exhibited a growth advantage under iron-deficient conditions and hypersensitivity to hydrogen peroxide in iron-rich medium. Transcription of <i>rirA</i> in HH103 is subject to autoregulation and inactivation of the gene upregulates <i>fbpA</i>, a gene putatively involved in iron transport. The <i>S. fredii rirA</i> 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 H<sub>2</sub>O<sub>2</sub>, expression of <i>fbpA</i>, and symbiotic deficiency in soybean plants. This is the first report that demonstrates a role for RirA in the <i>Rhizobium</i>-legume symbiosis.
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