Alkyl hydroperoxide reductase (ahpC) and organic hydroperoxide resistance (ohr) are distinct genes, structurally and regulatory, but have similar physiological functions. In Xanthomonas campestris pv. phaseoli inactivation of either gene results in increased sensitivity to killing with organic peroxides. An alipC1-ohr double mutant was highly sensitive to both growth inhibition and killing treatment with organic peroxides, High level expression of uhpC or ohr only partially complemented the phenotype of the double mutant, suggesting that these genes function synergistically, but through different pathways, to protect Xanthomonas from organic peroxide toxicity. Functional analyses of Ohr and AhpC abilities to degrade organic hydroperoxides revealed that both Ohr and AhpC could degrade tert-butyl hydroperoxide (tBOOH) awhile the former was more efficient at degrading cumene hydroperoxide (CuOOH). Expression analysis of these genes in the mutants showed no compensatory alterations in the levels of AhpC or Ohr. However, CuOOH induced expression of these genes in the Mutants vas affected. CuOOH induced ahpC expression was higher in the ohr mutant than in the parental strain: in contrast, the ahpC mutation has no effect on the level of induced ohr expression. These analyses reveal complex physiological roles and expression patterns of seemingly functionally similar genes. (C) 2002 Elsevier Science (USA). All rights reserved.