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Journal of Bacteriology, August 2001, p. 4405-4412, Vol. 183, No. 15
0021-9193/01/$04.00+0   DOI: 10.1128/JB.183.15.4405-4412.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

Complex Regulation of the Organic Hydroperoxide Resistance Gene (ohr) from Xanthomonas Involves OhrR, a Novel Organic Peroxide-Inducible Negative Regulator, and Posttranscriptional Modifications

Rojana Sukchawalit,1,dagger Suvit Loprasert,1 Sopapan Atichartpongkul,1 and Skorn Mongkolsuk1,2,*

Laboratory of Biotechnology, Chulabhorn Research Institute, Lak Si, Bangkok 10210,1 and Department of Biotechnology, Faculty of Science, Mahidol University, Bangkok 10400,2 Thailand

Received 14 February 2001/Accepted 1 May 2001

Analysis of the sequence immediate upstream of ohr revealed an open reading frame, designated ohrR, with the potential to encode a 17-kDa peptide with moderate amino acid sequence homology to the MarR family of negative regulators of gene expression. ohrR was transcribed as bicistronic mRNA with ohr, while ohr mRNA was found to be 95% monocistronic and 5% bicistronic with ohrR. Expression of both genes was induced by tert-butyl hydroperoxide (tBOOH) treatment. High-level expression of ohrR negatively regulated ohr expression. This repression could be overcome by tBOOH treatment. In vivo promoter analysis showed that the ohrR promoter (P1) has organic peroxide-inducible, strong activity, while the ohr promoter (P2) has constitutive, weak activity. Only P1 is autoregulated by OhrR. ohr primer extension results revealed three major primer extension products corresponding to the 5' ends of ohr mRNA, and their levels were strongly induced by tBOOH treatment. Sequence analysis of regions upstream of these sites showed no typical Xanthomonas promoter. Instead, the regions can form a stem-loop secondary structure with the 5' ends of ohr mRNA located in the loop section. The secondary structure resembles the structure recognized and processed by RNase III enzyme. These findings suggest that the P1 promoter is responsible for tBOOH-induced expression of the ohrR-ohr operon. The bicistronic mRNA is then processed by RNase III-like enzymes to give high levels of ohr mRNA, while ohrR mRNA is rapidly degraded.

* Corresponding author. Mailing address: Laboratory of Biotechnology, Chulabhorn Research Institute, Lak Si, Bangkok 10210, Thailand. Phone: (662) 574-0623, ext. 1402. Fax: (662) 574-2027. E-mail: skorn{at}tubtim.cri.or.th.

dagger Present address: School of Bioscience, University of Birmingham, Edgbaston, Birmingham, United Kingdom.

Journal of Bacteriology, August 2001, p. 4405-4412, Vol. 183, No. 15
0021-9193/01/$04.00+0   DOI: 10.1128/JB.183.15.4405-4412.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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