Abstract
BackgroundSinorhizobium meliloti is a symbiotic nitrogen-fixing bacterium that elicits nodules on roots of host plants Medicago sativa. During nodule formation bacteria have to withstand oxygen radicals produced by the plant. Resistance to H2O2 and superoxides has been extensively studied in S. meliloti. In contrast resistance to organic peroxides has not been investigated while S. meliloti genome encodes putative organic peroxidases. Organic peroxides are produced by plants and are highly toxic. The resistance to these oxygen radicals has been studied in various bacteria but never in plant nodulating bacteria.ResultsIn this study we report the characterisation of organic hydroperoxide resistance gene ohr and its regulator ohrR in S. meliloti. The inactivation of ohr affects resistance to cumene and ter-butyl hydroperoxides but not to hydrogen peroxide or menadione in vitro. The expression of ohr and ohrR genes is specifically induced by organic peroxides. OhrR binds to the intergenic region between the divergent genes ohr and ohrR. Two binding sites were characterised. Binding to the operator is prevented by OhrR oxidation that promotes OhrR dimerisation. The inactivation of ohr did not affect symbiosis and nitrogen fixation, suggesting that redundant enzymatic activity exists in this strain. Both ohr and ohrR are expressed in nodules suggesting that they play a role during nitrogen fixation.ConclusionsThis report demonstrates the significant role Ohr and OhrR proteins play in bacterial stress resistance against organic peroxides in S. meliloti. The ohr and ohrR genes are expressed in nodule-inhabiting bacteroids suggesting a role during nodulation.
Highlights
Sinorhizobium meliloti is a symbiotic nitrogen-fixing bacterium that elicits nodules on roots of host plants Medicago sativa
SMc00040 has been shown to be induced by peroxide stress [11]; it is divergently located from a gene encoding a MarR family regulator that has 49 and 45% identity with the OhrR regulatory protein of X. campestris and B. subtilis respectively
Co-localisation on the genome of ohr and ohrR was found in all bacteria in which these genes were investigated [20,31,36], suggesting that SMc00040 and SMc00098 encodes respectively Ohr and OhrR proteins
Summary
Sinorhizobium meliloti is a symbiotic nitrogen-fixing bacterium that elicits nodules on roots of host plants Medicago sativa. During nodule formation bacteria have to withstand oxygen radicals produced by the plant. Resistance to H2O2 and superoxides has been extensively studied in S. meliloti. Organic peroxides are produced by plants and are highly toxic. The resistance to these oxygen radicals has been studied in various bacteria but never in plant nodulating bacteria. Sinorhizobium meliloti is a soil bacterium that must survive and proliferate in various adverse conditions. S. meliloti is able to establish a symbiotic partnership with Medicago sativa leading to the formation of nodules. The bacterium differentiates in bacteroids and fixes atmospheric nitrogen. While nodulation is a close association between plant and S. meliloti, bacteria are initially recognised as intruders and induce an oxidative burst [4].
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