FIG 8.

Diagram showing regulation of the ferric (Fe³⁺) and ferrous (Fe²⁺) uptake systems in E. coli. Fur-Fe²⁺ is the master regulator of transcription of genes involved in iron metabolism. Under aerobic growth conditions, where Fe³⁺ is the major source of iron, E. coli secretes enterobactin (Ent) in the medium to chelate Fe³⁺. The Fe³⁺-chelate complex is transported back into the cell through the outer membrane receptor protein, FepA. The TonB-ExbB-ExbD complex of the inner membrane facilitates FepA channel opening. In the periplasm, FepB interacts with the Fe³⁺-chelate and delivers it to the FepDGC complex for transport into the cytoplasm. Under microaerobic or anaerobic growth conditions, Fe²⁺ is the main source of iron. It is brought into the cell via porins OmpC and OmpF and FeoB. The EnvZ/OmpR two-component system, classically known for regulating the expression of the ompC and ompF porin genes, also induces feo expression when hyperactivated due to a specific mutation in envZ (envZ_R397L). This positive effect of EnvZ_R397L/OmpR on feo expression can overcome the negative effect of Fur-Fe²⁺ on feo expression, thus tipping the balance in favor of ferrous over ferric transport. Porins and EnvZ/OmpR play a crucial role in iron acquisition in a TonB-deficient background that lacks functional ferric transport systems. Abbreviations: OM, outer membrane; PS, periplasm; IM, inner membrane; Cyt, cytoplasm; P, promoter.