Characterization of the Siderophore ofFrancisella tularensisand Role offslAin Siderophore Production

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Abstract
We determined that LVS and Schu S4 strains of the human pathogenFrancisella tularensisexpress a siderophore when grown under iron-limiting conditions. We purified this siderophore by conventional column chromatography and high-pressure liquid chromatography and used mass spectrometric analysis to demonstrate that it is structurally similar to the polycarboxylate siderophore rhizoferrin. The siderophore promoted the growth of LVS and Schu S4 strains in iron-limiting media. We identified a potential siderophore biosynthetic gene cluster encoded byfslABCDin theF. tularensisgenome. The first gene in the cluster,fslA, encodes a member of the superfamily of nonribosomal peptide synthetase-independent siderophore synthetases (NIS synthetases) characterized by the aerobactin synthetases IucA and IucC. We determined thatfslAis transcribed as part of an operon with downstream genefslBand that the expression of the locus is induced by iron starvation. A targeted in-frame nonpolar deletion offslAin LVS resulted in the loss of siderophore expression and in a reduced ability ofF. tularensisto grow under conditions of iron limitation. Siderophore activity and the ability to grow under iron limitation could be regained by introducing thefslA+gene on a complementing plasmid. Our results suggest that thefslA-dependent siderophore is important for survival ofF. tularensisin an iron-deficient environment.

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