IDENTIFICATION AND CHARACTERIZATION OF A NEW ORGANIC HYDROPEROXIDE RESISTANCE (OHR) GENE WITH A NOVEL PATTERN OF OXIDATIVE STRESS REGULATION FROM XANTHOMONAS-CAMPESTRIS PV. PHASEOLI

We have isolated a new organic hydroperoxide resistance (ohr) gene fro m Xanthomonas campestris pv. phase-oli. This was done by complementati on of an Escherichia coli alkyl hydroperoxide reductase mutant with an organic hydroperoxide-hypersensitive phenotype. ohr encodes a 14.5-kD a protein. Its amino acid sequence shows high homology with several pr oteins of unknown function. An ohr mutant was subsequently constructed , and it showed increased sensitivity to both growth-inhibitory and ki lling concentrations of organic hydroperoxides but not to either H2O2 or superoxide generators. No alterations in sensitivity to other oxida nts or stresses were observed in the mutant. ohr had interesting expre ssion patterns in response to low concentrations of oxidants. It was h ighly induced by organic hydroperoxides, weakly induced by H2O2, and n ot induced at all by a superoxide generator. The novel regulation patt ern of ohr suggests the existence of a second organic hy droperoxide-i nducible system that differs from the global peroxide regulator system , OxyR. Expression of ohr in various bacteria tested conferred increas ed resistance to tert-butyl hydroperoxide killing, but this was not so for wild-type Xanthomonas strains. The organic hydroperoxide hypersen sitivity of ohr mutants could be fully complemented by expression of o hr or a combination of ahpC and ahpF and could be partially complement ed by expression ahpC alone. The data suggested that Ohr was a new typ e of organic hydroperoxide detoxification protein.