Figure 5.

Mechanism of action of the M. xanthus CorE sigma factor. A. In the absence of copper, CorE remains inactive. B. When copper enters the cell, CorE is activated by binding this metal in its divalent oxidation state, initiating the transcription of genes involved in the immediate response to this metal. The immediate response includes the P_1B‐type ATPases CopA and CopB, which will extrude copper to the periplasm, and the multicopper oxidase CuoB, which will oxidize Cu⁺ to Cu²⁺ in the periplasm. C. Due to the strongly reducing environment of the cytoplasm, Cu²⁺ will be quickly reduced to Cu⁺, inactivating the ECF sigma factor (presumably via a conformational change), and stopping the immediate response even when copper is still present in the medium.