The DraC usher in Dr fimbriae biogenesis of uropathogenic E. coli Dr(+) strains.

Biogenesis of Dr fimbriae encoded by the dra gene cluster of uropathogenic Escherichia coli strains requires the chaperone-usher pathway. This secretion system is based on two non-structural assembly components, the DraB periplasmic chaperone and DraC outer-membrane usher. The DraB controls the folding of DraE subunits, and DraC forms the assembly and secretion platform for polymerization of subunits in linear fibers. In this study, mutagenesis of the DraC N-terminus was undertaken to select residues critical for Dr fimbriae bioassembly. The DraC-F4A, DraC-C64, DraC-C100A and DraC-W142A significantly reduced the adhesive ability of E. coli strains. The biological activity of the DraC mutants as a assembly platform for Dr fimbriae polymerization was verified by agglutination of human erythrocytes and adhesion to DAF localized at the surface of CHO-DAF(+) and HeLa cells. The residue F4 of the DraC usher conserved among FGL and FGS chaperone-assembled adhesive organelles can be used to design pillicides blocking the biogenesis of Dr fimbriae. Because the draC and afaC-III genes share 100% identity the range of the virulence determinant inhibitors could also be extended to E. coli strains encoding afa-3 gene cluster. The investigations performed showed that the usher N-terminus plays an important role in biogenesis of complete fiber.

A two-plasmid Escherichia coli system for expression of Dr adhesins.

This paper presents a very efficient expression system for production of Dr adhesins. The system consists of two plasmids. One is the pACYCpBAD-DraC-C-His, which contains the draC gene under the control of the arabinose promoter (pBAD), encoding the DraC usher. The second is the pET30b-syg-DraBE, which contains the draB and draE genes under the control of the T7lac promoter, encoding the DraB chaperone and the DraE adhesin, respectively. Those plasmids have different origin of replication and can therefore coexist in one cell. Since different promoters are present, the protein expression can be controlled. The Dr adhesion expression system constructed opens up a lot of possibilities, and could be very useful in experiments focusing on understanding the biogenesis of Gram-negative bacteria adhesins. For this purpose we showed that the AfaE-III adhesin (98.1% identity between the DraE and the AfaE-III adhesins, with three divergent amino acids within the sequences) was able to pass through the DraC channel in the Escherichia coli BL21(DE3) strain. Immunoblotting analysis and immunofluorescence microscopy showed the presence of AfaE-III on the bacterial cell surface. In addition, the system described can be useful for displaying the immune-relevant sectors of foreign proteins on the bacterial cell. The heterologous epitope sequence of the HSV1 glycoprotein D was inserted into the draE gene in place of the N-terminal region of surface exposed domain 2. Chimeric proteins were exposed on the bacterial surface as evidenced by immunoblotting and immunofluorescence microscopy. The effective display of peptide segments on Dr fimbriae expressed at the bacterial cell surface, can be used for the development of a fimbrial vaccine.