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Journal of Bacteriology, March 2003, p. 1734-1738, Vol. 185, No. 5
0021-9193/03/$08.00+0     DOI: 10.1128/JB.185.5.1734-1738.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

A Suppressor of the Menadione-Hypersensitive Phenotype of a Xanthomonas campestris pv. phaseoli oxyR Mutant Reveals a Novel Mechanism of Toxicity and the Protective Role of Alkyl Hydroperoxide Reductase

Laboratory of Biotechnology, Chulabhorn Research Institute, Bangkok 10210,¹ Department of Biotechnology, Faculty of Science, Mahidol University, Bangkok 10400, Thailand²

Received 16 September 2002/ Accepted 8 December 2002

We isolated menadione-resistant mutants of Xanthomonas campestris pv. phaseoli oxyR (oxyR_Xp). The oxyRR2_Xp mutant was hyperresistant to the superoxide generators menadione and plumbagin and was moderately resistant to H₂O₂ and tert-butyl hydroperoxide. Analysis of enzymes involved in oxidative-stress protection in the oxyRR2_Xp mutant revealed a >10-fold increase in AhpC and AhpF levels, while the levels of superoxide dismutase (SOD), catalase, and the organic hydroperoxide resistance protein (Ohr) were not significantly altered. Inactivation of ahpC in the oxyRR2_Xp mutant resulted in increased sensitivity to menadione killing. Moreover, high levels of expression of cloned ahpC and ahpF in the oxyR_Xp mutant complemented the menadione hypersensitivity phenotype. High levels of other oxidant-scavenging enzymes such as catalase and SOD did not protect the cells from menadione toxicity. These data strongly suggest that the toxicity of superoxide generators could be mediated via organic peroxide production and that alkyl hydroperoxide reductase has an important novel function in the protection against the toxicity of these compounds in X. campestris.