FepA- and TonB-dependent bacteriophage H8: receptor binding and genomic sequence

H8 is derived from a collection of Salmonella enterica serotype Enteritidis bacteriophage. Its morphology and genomic structure closely resemble those of bacteriophage T5 in the family Siphoviridae. H8 infected S. enterica serotypes Enteritidis and Typhimurium and Escherichia coli by initial adsorption to the outer membrane protein FepA. Ferric enterobactin inhibited H8 binding to E. coli FepA (50% inhibition concentration, 98 nM), and other ferric catecholate receptors (Fiu, Cir, and IroN) did not participate in phage adsorption. H8 infection was TonB dependent, but exbB mutations in Salmonella or E. coli did not prevent infection; only exbB tolQ or exbB tolR double mutants were resistant to H8. Experiments with deletion and substitution mutants showed that the receptor-phage interaction first involves residues distributed over the protein's outer surface and then narrows to the same charged (R316) or aromatic (Y260) residues that participate in the binding and transport of ferric enterobactin and colicins B and D. These data rationalize the multifunctionality of FepA: toxic ligands like bacteriocins and phage penetrate the outer membrane by parasitizing residues in FepA that are adapted to the transport of the natural ligand, ferric enterobactin. DNA sequence determinations revealed the complete H8 genome of 104.4 kb. A total of 120 of its 143 predicted open reading frames (ORFS) were homologous to ORFS in T5, at a level of 84% identity and 89% similarity. As in T5, the H8 structural genes clustered on the chromosome according to their function in the phage life cycle. The T5 genome contains a large section of DNA that can be deleted and that is absent in H8: compared to T5, H8 contains a 9,000-bp deletion in the early region of its chromosome, and nine potentially unique gene products. Sequence analyses of the tail proteins of phages in the same family showed that relative to pb5 (Oad) of T5 and Hrs of BF23, the FepA-binding protein (Rbp) of H8 contains unique acidic and aromatic residues. These side chains may promote binding to basic and aromatic residues in FepA that normally function in the adsorption of ferric enterobactin. Furthermore, a predicted H8 tail protein showed extensive identity and similarity to pb2 of T5, suggesting that it also functions in pore formation through the cell envelope. The variable region of this protein contains a potential TonB box, intimating that it participates in the TonB-dependent stage of the phage infection process.

Evidence of ball-and-chain transport of ferric enterobactin through FepA

The Escherichia coli iron transporter, FepA, has a globular N terminus that resides within a transmembrane beta-barrel formed by its C terminus. We engineered 25 cysteine substitution mutations at different locations in FepA and modified their sulfhydryl side chains with fluorescein maleimide in live cells. The reactivity of the Cys residues changed, sometimes dramatically, during the transport of ferric enterobactin, the natural ligand of FepA. Patterns of Cys susceptibility reflected energy- and TonB-dependent motion in the receptor protein. During transport, a residue on the normally buried surface of the N-domain was labeled by fluorescein maleimide in the periplasm, providing evidence that the transport process involves expulsion of the globular domain from the beta-barrel. Porin deficiency much reduced the fluoresceination of this site, confirming the periplasmic labeling route. These data support the previously proposed, but never demonstrated, ball-and-chain theory of membrane transport. Functional complementation between a separately expressed N terminus and C-terminal beta-barrel domain confirmed the feasibility of this mechanism.