A novel copper-sensing two-component system for activating Dsb genes in bacteria

Copper is an essential element for biological systems but becomes toxic when present in excess. In Pseudomonas aeruginosa, an important human pathogen, the resistance to copper requires the induction of dsbDEG operon encoding proteins involved in disulfide-bond formation (Dsb). However, it is unknown how the copper stress induces the transcription of the operon. Here, we report that the exogenous copper induces the transcription of the dsbDEG operon through a new copper-sensing two-component system named DsbRS. The dsbRS is divergently transcribed from the dsbDEG operon, and the response regulator DsbR binds to the intergenic region between the operons. In the absence of copper, the sensor kinase DsbS acts as a phosphatase toward DsbR and thus blocks the transcription of the operons. However, in the presence of copper, the metal ion directly binds to the sensor domain of DsbS, for which the Cys82 residue plays a critical role. The copper-binding appears to inhibit the phosphatase activity of DsbS, leading to activation of DsbR. The copper resistance of the dsbRS knock-out mutant was restored by ectopic expression of the dsbDEG operon, confirming the critical role of the operon in the resistance to copper. Strikingly, cognates of dsbRS-dsbDEG pair are widely distributed across eubacteria. Also, a DsbR-binding site is detected in the promoter region of dsbDEG homologs in those species. Thus, regulation of Dsb genes by DsbRS represents a novel mechanism by which bacterial cells cope with copper stress.