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FIG. 1. Schematic representation of iron acquisition systems in N. meningitidis. Under iron-limiting conditions, meningococci express several proteins involved in the uptake and processing of iron and heme. Several of these are outer membrane receptor proteins, which recognize transferrin, lactoferrin, hemoglobin, and hemoglobin-haptoglobin. Depicted are the TonB-dependent receptors, TbpAB, LbpAB, HmbR, and HpuAB. All consist of a pore-forming membrane protein and, except for HmbR, a lipoprotein that is involved in substrate binding. An uncharacterized TonB-independent heme uptake system is not shown. The FbpABC transport proteins are involved in the periplasmic transport of ferric iron, and an undescribed system may exist for the periplasmic transport of heme. In the cytoplasm, ferric iron and heme can be stored in bacterioferritin. Heme can also be converted to biliverdin and CO in the cytoplasm by heme oxygenase. Dimeric forms of TonB and the FbpBC proteins from E. coli have been described but are not depicted as such here. LOS, lipooligosaccharide

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