OhrR Is a Repressor of ohrA, a Key Organic Hydroperoxide Resistance Determinant in Bacillus subtilis

Bacillus subtilis displays a complex adaptive response to the presence of reactive oxygen species. To date, most proteins that protect against reactive oxygen species are members of the peroxide-inducible PerR and ς(B) regulons. We investigated the function of two B. subtilis homologs of the Xanthomonas campestris organic hydroperoxide resistance (ohr) gene. Mutational analyses indicate that both ohrA and ohrB contribute to organic peroxide resistance in B. subtilis, with the OhrA protein playing the more important role in growing cells. Expression of ohrA, but not ohrB, is strongly and specifically induced by organic peroxides. Regulation of ohrA requires the convergently transcribed gene, ohrR, which encodes a member of the MarR family of transcriptional repressors. In an ohrR mutant, ohrA expression is constitutive, whereas expression of the neighboring ohrB gene is unaffected. Selection for mutant strains that are derepressed for ohrA transcription identifies a perfect inverted repeat sequence that is required for OhrR-mediated regulation and likely defines an OhrR binding site. Thus, B. subtilis contains at least three regulons (ς(B), PerR, and OhrR) that contribute to peroxide stress responses.