Figure 1.

Routes of iron trafficking in bacterial cells. Heavy arrows depict intracellular movement of iron, light arrows show the movement of iron or iron-bearing compounds across the cell membrane, and lines connect the transcriptional regulators to systems under their control. When the concentration of the labile iron pool increases, iron, or an iron-containing group, binds to the transcriptional regulator. This leads to down-regulation of processes such as siderophore synthesis, export of apo-siderophores, import of Fe³⁺-siderophores, heme import, and Fe²⁺ uptake systems. Simultaneously, expression of iron-containing and iron storage proteins is up-regulated together, occasionally, with iron efflux pumps. Reduction in the labile iron pool leads to dissociation of iron/iron-containing groups from the regulators, resulting in the opposite transcriptional responses.