In gram-negative organisms, high-affinity transport of iron substrates requires energy transduction to specific outer membrane receptors by the TonB-ExbB-ExbD complex. Vibrio cholerae encodes two TonB proteins, one of which, TonB1, recognizes only a subset of V. cholerae TonB-dependent receptors and does not facilitate transport through Escherichia coli receptors. To investigate the receptor specificity exhibited by V. cholerae TonB1, chimeras were created between V. cholerae TonB1 and E. coli TonB. The activities of the chimeric TonB proteins in iron utilization assays demonstrated that the C-terminal one-third of either TonB confers the receptor specificities associated with the full-length TonB. Single-amino-acid substitutions near the C terminus of V. cholerae TonB1 were identified that allowed TonB1 to recognize E. coli receptors and at least one V. cholerae TonB2-dependent receptor. This indicates that the very C-terminal end of V. cholerae TonB1 determines receptor specificity. The regions of the TonB-dependent receptors involved in specificity for a particular TonB protein were investigated in experiments involving domain switching between V. cholerae and E. coli receptors exhibiting different TonB specificities. Switching the conserved TonB box heptapeptides at the N termini of these receptors did not alter their TonB specificities. However, replacing the amino acid immediately preceding the TonB box in E. coli receptors with an aromatic residue allowed these receptors to use V. cholerae TonB1. Further, site-directed mutagenesis of the TonB box -1 residue in a V. cholerae TonB2-dependent receptor demonstrated that a large hydrophobic amino acid in this position promotes recognition of V. cholerae TonB1. These data suggest that the TonB box -1 position controls productive interactions with V. cholerae TonB1.
Shigella flexneri possesses multiple iron acquisition systems, including proteins involved in the synthesis and uptake of siderophores and the Feo system for ferrous iron utilization. We identified an additional S. flexneri putative iron transport gene, sitA, in a screen for S. flexneri genes that are induced in the eukaryotic intracellular environment. sitA was present in all Shigella species and in most enteroinvasive Escherichia coli strains but not in any other E. coli isolates tested. The sit locus consists of four genes encoding a potential ABC transport system. The deduced amino acid sequence of the S. flexneri sit locus was homologous to the Salmonella enterica serovar Typhimurium Sit and Yersinia pestis Yfe systems, which mediate both manganese and iron transport. The S. flexneri sit promoter was repressed by either iron or manganese, and the iron repression was partially dependent upon Fur. A sitA::cam mutation was constructed in S. flexneri. The sitA mutant showed reduced growth, relative to the wild type, in Luria broth containing an iron chelator but formed wild-type plaques on Henle cell monolayers, indicating that the sitA mutant was able to acquire iron and/or manganese in the host cell. However, mutants defective in two of these iron acquisition systems (sitA iucD, sitA feoB, and feoB iucD) formed slightly smaller plaques on Henle cell monolayers. A strain carrying mutations in sitA, feoB, and iucD did not form plaques on Henle cell monolayers.
We determined the complete genome sequence of Shigella flexneri serotype 2a strain 2457T (4,599,354 bp). Shigella species cause >1 million deaths per year from dysentery and diarrhea and have a lifestyle that is markedly different from those of closely related bacteria, including Escherichia coli. The genome exhibits the backbone and island mosaic structure of E. coli pathogens, albeit with much less horizontally transferred DNA and lacking 357 genes present in E. coli. The strain is distinctive in its large complement of insertion sequences, with several genomic rearrangements mediated by insertion sequences, 12 cryptic prophages, 372 pseudogenes, and 195 S. flexneri-specific genes. The 2457T genome was also compared with that of a recently sequenced S. flexneri 2a strain, 301. Our data are consistent with Shigella being phylogenetically indistinguishable from E. coli. The S. flexneri-specific regions contain many genes that could encode proteins with roles in virulence. Analysis of these will reveal the genetic basis for aspects of this pathogenic organism's distinctive lifestyle that have yet to be explained.