FIGURE 2.

A simplified view of siderophore-meidated iron transport in Gram-negative bacteria. Siderophores are synthesized in the cytoplasm and exported by diverse exporters (Exp) on the IM (inner membrane) and OM (outer membrane). Ferrisiderophores are imported into the periplasm by TonB-dependent receptor (TBDR) on the OM. In most cases, ferrisiderophores are subsequently transported by transporters, mostly ABC transporters (T), into the cytoplasm, where reduction takes place by a specific ferrisiderophore reductase (FSR) to release iron in the ferrous form. In some rare cases, ferrisiderophores in the periplasm are directly reduced to release ferrous iron and siderophore molecules, which then are transported across the IM by a specific ABC transporter and across the OM by siderophore exporter for reuse respectively.