Identification of Francisella tularensis genes affected by iron limitation

Cells of an attenuated live vaccine strain (LVS) of F. tularensis grown under iron-restricted conditions were found to contain increased quantities of several proteins relative to cells of this same strain grown under iron-replete conditions. Mass spectrometric analysis identified two of these proteins as IglC and PdpB, both of which are encoded by genes located in a previously identified pathogenicity island in F. tularensis LVS. Regions with homology to the consensus Fur box sequence were located immediately in front of the iglC and pdpB open reading frames (ORFs), and in silico analysis of the F. tularensis Schu4 genome detected a number of predicted 5′ untranslated regions that contained putative Fur boxes. The putative Fur box preceding Francisella iron-regulated gene A (figA) had the highest degree of identity with the consensus Fur box sequence. DNA microarray analysis showed that nearly 80 of the genes in the F. tularensis LVS genome were up- or down-regulated at least twofold under iron-restricted growth conditions. When tested for possible siderophore production by means of the Chrome Azurol S assay, a wild-type F. novicida strain produced a large reaction zone whereas its figA mutant produced very little reactivity in this assay. In addition, a cross-feeding experiment demonstrated that this siderophore-like activity produced by the wild-type F. novicida strain could enhance the ability of the F. novicida figA mutant to grow under iron-restricted conditions. This study provides the first identification of iron-regulated genes in F. tularensis LVS and evidence for the production of a siderophore-like molecule by F. novicida.