Protection of Xanthomonas against arsenic toxicity involves the peroxide-sensing transcription regulator OxyR

Abstract

Arsenic has been shown to mediate its toxicity through induced generation of reactive oxygen species. Here, we examined the role of oxidative stress-inducible genes ( katA, ahpC and ohr) and their regulators ( oxyR and ohrR) in the response to arsenic treatment in a plant pathogenic bacterium, Xanthomonas campestris pv. phaseoli ( Xp). Overproduction of peroxide-scavenging enzymes (KatA, AhpCF and Ohr) did not enhance arsenic tolerance in wild-type Xp. Furthermore, inactivation of katA, ahpC, ohr, and ohrR genes had no effect on the level of arsenic resistance. By contrast, an oxyR mutant ( Xp oxyR) showed increased sensitivity to both pentavalent arsenate and, to a greater extent, trivalent arsenite. The resistance of cells to arsenite treatment was significantly affected by the level of iron. Cells were 10-fold more sensitive to arsenite killing in the presence of excess iron, while removal of iron by an iron chelator (2,2′-dipyridyl) protected Xanthomonas from arsenite toxicity. The arsenite-sensitive phenotype of Xp oxyR could be complemented by the expression of functional OxyR from a plasmid vector, but not by the expression of other known OxyR-regulated peroxide-scavenging enzymes such as KatA and AhpCF, Ohr and OhrR. The data suggested that as yet unidentified, OxyR-regulated gene(s) are involved in conferring arsenic resistance in Xp. To our knowledge, this is the first report showing that the peroxide-sensing regulator OxyR is involved in arsenic resistance.