Identification of a regulatory pathway that controls the heavy-metal resistance system Czc via promoter czcNp in Ralstonia metallidurans

The CzcCBA cation-proton-antiporter is the most complicated and efficient heavy-metal resistance system known today and is essential for survival of Ralstonia metallidurans at high cobalt, zinc, or cadmium concentrations. Expression of Czc is tightly controlled by the complex interaction of several regulators. Double- and multiple-deletion studies demonstrated that four regulators encoded downstream of the czcCBA operon, CzcD, CzcS, CzcR and the newly identified CzcE, are involved in, but not essential for metal-dependent induction of czc. These proteins control expression of the czcNICBA region from the promoter czcNp. Northern analysis showed that czcDRS was transcribed as czcDR-specific and czcDRS-specific mRNAs. Transcription of czcE occurred independently of czcDRS transcription and was induced by zinc. CzcE is a periplasmic protein as indicated by phoA fusions. CzcE was purified and identified as a metal-binding protein. These data demonstrate that the transport protein CzcD, the two-component regulatory system CzcR, CzcS, and the periplasmic metal-binding protein CzcE exert metal-dependent control of czcNICBA expression via regulation of czcNp activity.