A critical role of the periplasm in copper homeostasis in Gram-negative bacteria.

Copper is essential for life, but is toxic in excess. Copper homeostasis is achieved in the cytoplasm and the periplasm as a unique feature of Gram-negative bacteria. Especially, it has become clear the role of the periplasm and periplasmic proteins regarding whole-cell copper homeostasis. Here, we addressed the role of the periplasm and periplasmic proteins in copper homeostasis using a Systems Biology approach integrating experiments with models. Our analysis shows that most of the copper-bound molecules localize in the periplasm but not cytoplasm, suggesting that Escherichia coli utilizes the periplasm to sense the copper concentration in the medium and sequester copper ions. In particular, a periplasmic multi-copper oxidase CueO and copper-responsive transcriptional factor CusS contribute both to protection against Cu(I) toxicity and to incorporating copper into the periplasmic components/proteins. We propose that Gram-negative bacteria have evolved mechanisms to sense and store copper in the periplasm to expand their living niches.