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J. Bacteriol. doi:10.1128/JB.00632-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Dual Role of OhrR as a Repressor and an Activator in Response to Organic Hydroperoxides in Streptomyces coelicolor

Laboratory of Molecular Microbiology, School of Biological Sciences, and Institute of Microbiology, Seoul National University, Seoul, 151-742, Korea

^* To whom correspondence should be addressed. Email: jhroe{at}snu.ac.kr.

	Abstract

Organic hydroperoxide resistance (Ohr) in bacteria is achieved primarily through reducing oxidized membrane lipids. The soil-inhabiting aerobic bacteria Streptomyces coelicolor contains three paralogous genes for Ohr; ohrA, ohrB, and ohrC. The ohrA gene is transcribed divergently from ohrR encoding a putative regulator of MarR family. Both ohrA and ohrR genes were induced highly by various organic hydroperoxides. The ohrA gene was induced through removing repression by OhrR whereas the ohrR gene was induced through activation by OhrR. Reduced OhrR bound to the ohrA/ohrR intergenic region, which contains a central (primary) and two adjacent (secondary) inverted repeat motifs that overlap with promoter elements. Organic peroxide decreased the binding affinity of OhrR to the primary site, accompanied by a concomitant decrease in cooperative binding to the adjacent secondary sites. The single cysteine C28 in OhrR was involved in sensing oxidants, as judged by substitution mutagenesis. The C28S mutant OhrR bound to the intergenic region without any change in binding affinity in response to organic peroxides. These results lead us to propose a model for the dual action of OhrR as a repressor and an activator in S. coelicolor. Under reduced condition OhrR binds cooperatively to the intergenic region, repressing transcription from both genes. Upon oxidation, the binding affinity of OhrR decreases with a concomitant loss of cooperative binding, which allows RNA polymerase to bind to both ohrA and ohrR promoters, and the loosely bound oxidized OhrR can further activate transcription from the ohrR promoter.

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