An ABC Transporter System of Yersinia pestis Allows Utilization of Chelated Iron by Escherichia coli SAB11

Yersinia pestis is the facultative intracellular, gram-negative bacterium that causes bubonic and pneumonic plague. In nature, the organism exists as a zoonotic disease of rodents and their associated fleas and has been responsible for the widespread loss of human life during several catastrophic pandemics (59). Colonization and growth of bacterial pathogens, including Y. pestis, are dependent on the ability of the invading organism to acquire iron. Since most of the iron in a mammalian host is complexed with metalloproteins or sequestered by the iron storage protein ferritin, bacteria have evolved elaborate strategies to overcome this iron-deficient environment in order to obtain iron for growth (37, 54). Many utilize a variety of extracellular, low-molecular-weight ferric iron chelators termed siderophores (15, 36, 37). Still others obtain iron directly from hemoproteins or the high-affinity iron-binding glycoproteins transferrin and lactoferrin through specific receptors on the bacterial surface (22, 35, 51, 54). The relationship between iron availability and virulence was clearly demonstrated in spontaneous nonpigmented (Pgm−) mutants of Y. pestis, in which virulence in mice was restored when the strains were inoculated via peripheral routes and supplemented with hemin or inorganic iron (49). Iron-independent virulence in mice and formation of dark brown, or pigmented, colonies at 26°C from adsorption of exogenous hemin were the original defining characteristics of the chromosomally encoded pigmentation (Pgm+) phenotype of Y. pestis (48, 49). Pgm− mutants often arise via spontaneous deletion of the 102-kb pigmentation (pgm) locus, which is known to encode an ∼7-kb hemin storage (hms) locus required for the Hms+ phenotype (pigmented colony formation) and the yersiniabactin-iron transport system (Ybt) (59). The Ybt transport system is a siderophore-dependent uptake mechanism that is common to the three pathogenic species of Yersinia: Y. pestis, Yersinia pseudotuberculosis, and Yersinia enterocolitica (18, 32, 43, 59, 83). An ∼23-kb region possesses at least four genes likely encoding enzymes for the biosynthesis of the Ybt siderophore (9, 38) and for the receptor (Psn/FyuA) for Ybt and the bacteriocin pesticin (30, 31, 43, 61). Expression of Psn and the Ybt biosynthetic genes is negatively regulated by the Fur repressor but is also activated by YbtA (an AraC-type transcriptional activator located immediately upstream of the Ybt biosynthetic operon) and by the Ybt siderophore (29, 30, 43, 72, 74). Mutations in the Ybt transport system cause a drastic, or possibly complete, loss of virulence in mice infected subcutaneously (9). However, Y. pestis cells possess additional independent hemoprotein and iron transport systems. A hemin utilization system (Hmu) allows the use of hemin and hemoproteins (46, 58, 69, 73). In addition to the Ybt system, inorganic iron transport is mediated by at least two other separate mechanisms. First, studies which monitored the growth of Pgm− strains in the presence of iron chelators suggest that Y. pestis cells possess an iron transport system that is functional at 26°C but not at 37°C (52). Second, Ybt− mutants of Y. pestis retain the ability to grow at 37°C under iron-deficient but not iron-chelated conditions (9, 30, 58, 68, 69). In this study, we have used cloning and transposon mutagenesis to identify a pgm-independent transport system (Yfe) encoded in two distinct operons (yfeABCD and yfeE) and located on the Y. pestis chromosome. The data presented here suggest that the yfe locus of Y. pestis is an ATP-binding cassette (ABC) transport system involved in the acquisition of inorganic iron.