The Evolution and Sequence Diversity of FhuA in Salmonella and Escherichia

ThefhuACDBoperon, present in a number ofEnterobacteriaceae, encodes components essential for the uptake of ferric hydroxamate type siderophores. FhuA acts not only as transporter for physiologically important chelated ferric iron, but also as receptor for various bacteriophages, toxins and antibiotics, which are pathogenic to bacterial cells. In this research, thefhuAgene distribution and sequence diversity were investigated inEnterobacteriaceae, especiallySalmonellaandEscherichia. Comparative sequence analysis resulted in afhuAphylogenetic tree that did not match the expected phylogeny based on housekeeping sequence analysis or trees offhuCDBgenes. ThefhuAsequences showed a unique mosaic-clustering pattern. On the other hand, the gene sequences showed high conservation for strains from the same serovar or serotype. In total, six clusters were identified from FhuA proteins inSalmonellaandEscherichia, among which typical peptide fragment variations could be defined. Six fragmental insertions / deletions and two substitution fragments were discovered, which could well classify the different clusters. Structure modeling demonstrated that all the six featured insertions/deletions and one substitution fragment are located at the apexes of the long loops of FhuA external pocket. These frequently mutated regions are likely under high selection pressure, and bacterial strains could have escaped from phage infection or toxin / antibiotics attack viafhuAgene mutations while maintaining the siderophore uptake activity essential for bacterial survival. The unusualfhuAclustering suggests that high frequency exchange offhuAgenes has occurred between enterobacterial strains after distinctive species were established.IMPORTANCEThe enterobacterialfhuACDBoperon encodes proteins which mediate the uptake of siderophores to supply the cells with iron essential for bacterial survival. Here we show different evolutionary patterns for thefhugenes within the same operon. ThefhuAhas a phylogenetic tree that does not match the species phylogeny, whereas the rest of thefhugenes do. ThefhuAgenes showed inter-species sequence convergence and conservation within specific serovars and serotypes. Nearly all of the significant sequence differences among FhuA clusters are located in potential ligand-binding sites on the extracellular surface of fhuA-encoding receptors. The unusualfhuAclustering suggests the frequent recombination and exchange offhuAgenes between enterobacterial strains in the evolutionary state after distinctive species were established.Our findings suggested either a new evolutionary mechanism or local gene recombination infhuAthat is in contrast to previous evolutionary hypotheses that have formed under the assumption of no recombination.