The 58-Kilodalton Major Virulence Factor of Francisella tularensis Is Required for Efficient Utilization of Iron

We investigated the role of the 58-kDa FTT0918 protein in the iron metabolism of Francisella tularensis. The phenotypes of SCHU S4, a prototypic strain of F. tularensis subsp. tularensis, and the FTT0918 and fslA isogenic mutants were analyzed. The gene product missing in the fslA mutant is responsible for synthesis of a siderophore. When grown in broth with various iron concentrations, the two deletion mutants generally reached lower maximal densities than SCHU S4. The FTT0918 mutant, but not the fslAmutant, upregulated the genes of the F. tularensis siderophore locus (fsl) operon even at high iron concentrations. A chrome azurol sulfonate plate assay confirmed siderophore production by all strains except the fslA strain. In a cross-feeding experiment using medium devoid of free iron, SCHU S4 promoted growth of the fslA strain but not of the FTT0918 strain. The sensitivity of SCHU S4 and the FTT0918 and fslA strains to streptonigrin demon-strated that the FTT0918 strain contained a smaller free intracellular iron pool and that the fslA strain contained a larger one than SCHU S4. In contrast to the marked attenuation of the FTT0918 strain, the fslA strain was as virulent as SCHU S4 in a mouse model. Altogether, the data demonstrate that the FTT0918 protein is required for F. tularensis to utilize iron bound to siderophores and that it likely has a role also in siderophore-independent iron acquisition. We suggest that the FTT0918 protein be designated Fe utilization protein A, FupA