Biosynthesis of a Ysu siderophore has not been proven, and a siderophore biosynthetic pseudogene precedes the y2633-y2637 locus [18]. The OM β-barrel ferrichrome receptor FcuA#103 (Y2556) was identified as a protein of moderate check details abundance in usb-MBR fractions at 26°C (Figure 3) and 37°C, but not significantly altered in abundance comparing -Fe vs. +Fe conditions. Many membrane proteins ascribed to have putative functions in iron transport were not detected, e.g. the OM receptors Y3948

and IutA/Y3385 and the transport systems FitA-D (Y4043-Y4046), Y2837-Y2842 and FepB/Y3477. Our data support the notion of a hierarchy MCC950 clinical trial of iron (Fe3+)/siderophore transporters [15], with the Ybt and Yfe systems being dominant compared to the Yfu, Yiu and Hmu systems. Periplasmic subunits of two ferrous iron (Fe2+) transporters, EfeO/Y2451 and Y2368, were also profiled in 2D gels (Figure 1). The low Mr protein Y2368#72 was increased in iron-starved cells at 37°C. The tripartite Fe2+ transport protein EfeO#77 was increased in abundance in iron-starved cells at 26°C. The energy metabolism of Y. pestis is affected by iron starvation Lower growth rates of Y. pestis in deferrated medium followed by growth arrest at OD600s between 0.5 and 0.9 suggest perturbations of energy generation pathways. Many oxidoreductive processes are catalyzed by enzymes containing Fe-S clusters or heme, and we sought to understand

the consequences of limited iron availability as it pertains to the Y. pestis energy metabolism. The EcoCyc database http://​www.​ecocyc.​org click here with its extensive data on E. coli energy metabolic pathways and iron cofactors of proteins was a useful resource in this context. Y. pestis aconitases A and aminophylline B (AcnA#34 and AcnB#8; Figure

4) have functions in the TCA cycle and were decreased in abundance or detected only in iron-starved cells. So were subunits of two other TCA cycle enzymes harboring Fe-S clusters (SdhA#43 and FumA#11; Figure 4). Some TCA cycle enzymes devoid of Fe-S clusters were decreased at moderate levels under -Fe conditions (IcdA#26, SucA#42, SucD#41 and SucB#111; Figure 4). Strongly decreased abundances were denoted for AceA#2 and AceB#1 (Figure 4), enzymes which catalyze the glyoxalate bypass reaction of the TCA cycle and are regulated by the catabolite repressor protein (CRP). Glycerol kinase, also regulated by CRP, was more moderately decreased in iron-starved cells (GlpK#3, Figure 4). GlpK catalyzes the rate-limiting step of glycerol utilization and feeds its metabolites into the glycolytic pathway. CRP#91 itself was identified with low abundance in the periplasmic fraction (Figure 2). In summary, the data suggested reduced pyruvate metabolism via the citrate cycle when iron resources are exhausted in Y. pestis cells. Aconitase activity assays supported this assumption; the reaction rates were 2.