Fig. 1.

Y. pestis iron transport systems that are proven functional in vitro. These include a heme transporter (Hmu), three ferrous transporters (Yfe, Feo, and, Fet), three “ferric” transporters (Yfe, Yfu, and Yiu), one siderophore-dependent system (Ybt) fully functional in all Y. pestis biotypes, one system (Fhu/IutA/Fcu) capable of transporting, but not synthesizing aerobactin and ferrichrome bound to Fe (FeAero and FeFc). Finally, the Ysu system of Y. pestis CO92 (orientalis biotype) synthesizes and transports the Yersiniachelin (Ych) siderophore while Y. pestis KIM and Antiqua (medaevalis and antiqua biotypes) have a frameshift mutation that disrupts Ych synthesis. Although the fitABC and efeUOB loci are intact, their functionality remains to be tested. All OM receptors shown are TonB-dependent. FetMP requires one or more of the flp gene products (Y2363-Y2367) to be functional; for simplicity only Y2367 is shown. The Yfe ABC transporter can utilize ferric iron as a substrate, but its oxidation state during transport is undetermined. Dashed arrows in all figures indicate unknown components or unconfirmed steps.