Strains of Francisella tularensis secrete a siderophore in response to iron limitation. FslE belongs to a family of proteins that has no known homologs outside of the Francisella species and the fslE gene product has been shown to be localized on the outer membrane of F. tularensis. Experimental data suggest that FslE may function as the siderophore receptor in F. tularensis. Using a plate-based growth assay, it was found that the mutant was able to secrete a siderophore but was defective in the utilization of the siderophore. Prediction methods suggest that the ß-barrel domain is localized to the C-terminus of the protein similar to the family of TonB-dependent receptors.