fslE is necessary for siderophore-mediated iron acquisition in Francisella tularensis Schu S4.
Strains of Francisella tularensis secrete a siderophore in response to iron limitation. Siderophore production is dependent on fslA, the first gene in an operon that appears to encode biosynthetic and export functions for the siderophore. Transcription of the operon is induced under conditions of iron limitation. The fsl genes lie adjacent to the fur homolog on the chromosome, and there is a canonical Fur box sequence in the promoter region of fslA. We generated a Deltafur mutant of the Schu S4 strain of F. tularensis tularensis and determined that siderophore production was now constitutive and no longer regulated by iron levels. Quantitative reverse transcriptase PCR analysis with RNA from Schu S4 and the mutant strain showed that Fur represses transcription of fslA under iron-replete conditions. We determined that fslE (locus FTT0025 in the Schu S4 genome), located downstream of the siderophore biosynthetic genes, is also under Fur regulation and is transcribed as part of the fslABCDEF operon. We generated a defined in-frame deletion of fslE and found that the mutant was defective for growth under iron limitation. Using a plate-based growth assay, we found that the mutant was able to secrete a siderophore but was defective in utilization of the siderophore. FslE belongs to a family of proteins that has no known homologs outside of the Francisella species, and the fslE gene product has been previously localized to the outer membrane of F. tularensis strains. Our data suggest that FslE may function as the siderophore receptor in F. tularensis.
Department of Medicine, School of Medicine Core Facilities, University of Virginia, Charlottesville, Virginia 22908-5621, USA. girija@virginia.edu
Girija
G
Ramakrishnan
Alexis
A
Meeker
Bojan
B
Dragulev
D001425:Bacterial Outer Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D001426:Bacterial Proteins / Q000235:genetics; D015153:Blotting, Western; D005604:Francisella tularensis / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*; D017353:Gene Deletion; D020869:Gene Expression Profiling; D015964:Gene Expression Regulation, Bacterial; D005816:Genetic Complementation Test; D007501:Iron / Q000378:metabolism*; D011956:Receptors, Cell Surface / Q000235:genetics / Q000378:metabolism*; D012097:Repressor Proteins / Q000235:genetics; D017262:Siderophores / Q000378:metabolism*
D001425:Bacterial Outer Membrane Proteins; D001426:Bacterial Proteins; D011956:Receptors, Cell Surface; D012097:Repressor Proteins; D017262:Siderophores; C068477:ferric uptake regulating proteins, bacterial; C035902:siderophore receptors; D007501:Iron