The transcriptional response to cadmium, organic hydroperoxide, singlet oxygen and UV-A mediated by the σE-ChrR system inCaulobacter crescentus

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Caulobacter crescentusσE belongs to the ECF (extracytoplasmicfunction) subfamily of RNA polymerase sigma factors, whose members regulate gene expression in response to distinct environmental stresses. During physiological growth conditions, data indicate that σE is maintained in reduced levels due to the action of ChrR, a negative regulator ofrpoEgene expression and function. However, once bacterial cells are exposed to cadmium, organic hydroperoxide, singlet oxygen or UV-A irradiation, transcription ofrpoEis induced in a σE-dependent manner. Site-directed mutagenesis indicated that residue C188 in ChrR is critical for the cadmium response while residues H140 and H142 are required for the bacterial response to organic hydroperoxide, singlet oxygen and UV-A. Global transcriptional analysis showed that σE regulates genes involved in protecting cells against oxidative damages. A combination of transcriptional start site identification and promoter prediction revealed that some of these genes contain a putative σE-dependent motif in their upstream regions. Furthermore, deletion ofrpoEand two σE-dependent genes (cfaSandhsp20) impairsCaulobactersurvival when singlet oxygen is constantly generated in the cells.

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