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 r eactive oxygen species. To date, most proteins that protect against reactiv e oxygen species are members of the peroxide-inducible PerR and sigma (B) r egulons. We investigated the function of two B. subtilis homologs of the Xa nthomonas campestris organic hydroperoxide resistance (ohr) gene. Mutationa l 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 s pecifically induced by organic peroxides. Regulation of ohrA requires the c onvergently transcribed gene, ohrR, which encodes a member of the MarR fami ly of transcriptional repressors. In an ohrR mutant, ohrA expression is con stitutive, whereas expression of the neighboring ohrB gene is unaffected. S election for mutant strains that are derepressed for ohr A transcription id entifies a perfect inverted repeat sequence that is required for OhrR-media ted regulation and likely defines an OhrR binding site. Thus, B. subtilis c ontains at least three regulons (sigma (B), PerR, and OhrR) that contribute to peroxide stress responses.