title: Resistance to organic hydroperoxides requires ohr and ohrR genes in Sinorhizobium meliloti.
abstract: BACKGROUND
  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. 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.
  
  
  RESULTS
  In 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.
  
  
  CONCLUSIONS
  This 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.
authors.affiliation: UMR CNRS 6026, DUALS, Université de Rennes I, Campus de Beaulieu, Av. du Général Leclerc, 35042 Rennes, France.
authors.forename: Catherine
authors.initials: C
authors.lastname: Fontenelle
authors.forename: Carlos
authors.initials: C
authors.lastname: Blanco
authors.forename: Morgane
authors.initials: M
authors.lastname: Arrieta
authors.forename: Virginie
authors.initials: V
authors.lastname: Dufour
authors.forename: Annie
authors.initials: A
authors.lastname: Trautwetter
mesh_terms: D000900:Anti-Bacterial Agents / Q000633:toxicity*; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism*; D001665:Binding Sites; D004269:DNA, Bacterial / Q000378:metabolism; D024881:Drug Resistance, Bacterial; D017353:Gene Deletion; D055786:Gene Knockout Techniques; D009930:Organic Chemicals / Q000633:toxicity*; D010545:Peroxides / Q000633:toxicity*; D011485:Protein Binding; D012097:Repressor Proteins / Q000235:genetics / Q000378:metabolism; D016962:Sinorhizobium meliloti / Q000187:drug effects* / Q000235:genetics*
chemical_list: D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D004269:DNA, Bacterial; D009930:Organic Chemicals; D010545:Peroxides; D012097:Repressor Proteins