Ramakrishnan Girija, Meeker Alexis, Dragulev Bojan
Department of Medicine, School of Medicine Core Facilities, University of Virginia, Charlottesville, Virginia 22908-5621, USA.
J Bacteriol. 2008 Aug;190(15):5353-61. doi: 10.1128/JB.00181-08. Epub 2008 Jun 6.
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.
土拉弗朗西斯菌菌株在铁限制条件下会分泌一种铁载体。铁载体的产生依赖于fslA，它是一个操纵子中的第一个基因，该操纵子似乎编码铁载体的生物合成和输出功能。在铁限制条件下，该操纵子的转录被诱导。fsl基因位于染色体上与fur同源物相邻的位置，并且在fslA的启动子区域有一个典型的Fur框序列。我们构建了土拉弗朗西斯菌Schu S4菌株的Δfur突变体，并确定现在铁载体的产生是组成型的，不再受铁水平的调节。对来自Schu S4和突变菌株的RNA进行定量逆转录PCR分析表明，在铁充足的条件下，Fur抑制fslA的转录。我们确定位于铁载体生物合成基因下游的fslE（Schu S4基因组中的FTT0025位点）也受Fur调控，并作为fslABCDEF操纵子的一部分进行转录。我们构建了fslE的一个明确的框内缺失突变体，发现该突变体在铁限制条件下生长存在缺陷。使用基于平板的生长试验，我们发现该突变体能够分泌铁载体，但在铁载体的利用方面存在缺陷。FslE属于一个蛋白质家族，在弗朗西斯菌属之外没有已知的同源物，并且fslE基因产物先前已定位到土拉弗朗西斯菌菌株的外膜。我们的数据表明，FslE可能作为土拉弗朗西斯菌中的铁载体受体发挥作用。
