title: Characterization of the siderophore of Francisella tularensis and role of fslA in siderophore production.
abstract: We determined that LVS and Schu S4 strains of the human pathogen Francisella tularensis express 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 by fslABCD in the F. tularensis genome. 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 that fslA is transcribed as part of an operon with downstream gene fslB and that the expression of the locus is induced by iron starvation. A targeted in-frame nonpolar deletion of fslA in LVS resulted in the loss of siderophore expression and in a reduced ability of F. tularensis to grow under conditions of iron limitation. Siderophore activity and the ability to grow under iron limitation could be regained by introducing the fslA(+) gene on a complementing plasmid. Our results suggest that the fslA-dependent siderophore is important for survival of F. tularensis in an iron-deficient environment.
authors.affiliation: University of Virginia Health System, MR4 Bldg., Rm. 2126, P.O. Box 801367, Charlottesville, VA 22908-5621, USA.
authors.forename: Jonathan Tabb
authors.initials: JT
authors.lastname: Sullivan
authors.forename: Erin Field
authors.initials: EF
authors.lastname: Jeffery
authors.forename: John D
authors.initials: JD
authors.lastname: Shannon
authors.forename: Girija
authors.initials: G
authors.lastname: Ramakrishnan
mesh_terms: D001483:Base Sequence; D002851:Chromatography, High Pressure Liquid; D017931:DNA Primers; D005290:Ferric Compounds / Q000378:metabolism; D005604:Francisella tularensis / Q000254:growth & development / Q000378:metabolism*; D007501:Iron / Q000378:metabolism; D007700:Kinetics; D013058:Mass Spectrometry; D008969:Molecular Sequence Data; D016133:Polymerase Chain Reaction; D017262:Siderophores / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism*
chemical_list: D017931:DNA Primers; D005290:Ferric Compounds; D017262:Siderophores; C087496:rhizoferrin; D007501:Iron