ABSTRACT
Bacteria are exposed to varying concentrations of metals in their surroundings. In an event of depletion or accumulation of metals to toxic levels, bacteria activate cellular responses to maintain metal homeostasis. The expression of genes encoding metallochaperones, metal importers, and exporters is controlled by a panel of specialized transcriptional regulators known as metalloregulatory proteins or “metal sensor” proteins. In prokaryotes, the cellular response to perturbations in metal homeostasis is nearly exclusively transcriptional. The most functionally diverse group of metalloregulatory proteins is the ArsR/SmtB family of transcriptional repressors that regulate genes involved in effluxing, scavenging, or detoxifying excess metal ions found in the cytosol. The ferric uptake regulator (Fur) family of metalloregulatory proteins control transport and homeostasis of Fe2+, Zn2+, Mn2+, and Ni2+. Bacteria have evolved various metalloregulatory mechanisms for maintaining metal ion homeostasis which also participate in conferring metal resistance to overcome toxicity exerted by an increased metal ion concentration.
