Role of the Pseudomonas aeruginosa oxyR-recG Operon in Oxidative Stress Defense and DNA Repair: OxyR-Dependent Regulation of katB-ankB , ahpB , and ahpC-ahpF

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Abstract
Pseudomonas aeruginosa possesses an extensive armament of genes involved in oxidative stress defense, includingkatB-ankB, ahpB, and ahpC-ahpF. Transcription of these genes was regulated in response to H2O2, paraquat, or organic peroxides. Expression of katB-lacZ and the observed KatB catalase levels in P. aeruginosa PAO1 were induced up to 250-fold after exposure to oxidative stress-generating compounds. Also,ahpB-lacZ and ahpC-lacZ expression was 90- and 3-fold higher, respectively, upon exposure to paraquat. The dose- and time-response curves revealed that 1 μM paraquat was sufficient for half-maximal activation of each reporter fusion within 5 min of exposure. Expression of these genes was not observed in a ΔoxyR mutant, indicating that OxyR was essential for this response. The transcriptional start sites of katB-ankB,ahpB, and ahpC-ahpF were mapped, putative OxyR-binding sites were identified upstream of the −35 promoter elements, and direct binding of purified OxyR protein to these target promoters was demonstrated. The oxyR mutant was hypersusceptible to oxidative stress-generating agents, including H2O2 and paraquat, in spite of total KatA catalase activity being comparable to that of the wild type. TheoxyR phenotype was fully complemented by a plasmid containing the oxyR gene, while any of thekatB, ahpB, or ahpCF genes alone resulted in only marginal complementation. IncreasedkatB-lacZ expression and higher KatB catalase levels were detected in a ΔahpCF background compared to wild-type bacteria, suggesting a compensatory function for KatB in the absence of AhpCF. In P. aeruginosa, oxyR is located upstream of recG, encoding a putative DNA repair enzyme.oxyR-lacZ and recG-lacZ reporter activities andoxyR-recG mRNA analysis showed that oxyR andrecG are organized in an operon and expressed constitutively with regard to oxidative stress from a single promoter upstream of oxyR. Mutants affected in recG but not oxyR were dramatically impaired in DNA damage repair as measured by sensitivity to UV irradiation. In conclusion, we present evidence that the oxyR-recG locus is essential for oxidative stress defense and for DNA repair.
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