Comparative biochemical studies on F and EDP208 conjugative pili

EDP208 pili are encoded by a derepressed derivative of a naturally occurring lac plasmid, F(0)lac (incompatibility group FV), originally isolated from Salmonella typhi. EDP208 pili are serologically unrelated to F pili and do not promote infection by F-specific ribonucleic acid bacteriophages. However, they do confer sensitivity to the F-specific filamentous deoxyribonucleic acid phages. EDP208-containing cells are multi-piliated and contain approximately 20 pili per cell. These pili contain a single polypeptide subunit of 11,500 daltons. EDP208-specific RNA phages were readily isolated from local sewage. These phages were somewhat smaller in diameter than the F-specific ribonucleic acid phages and absorbed relatively weakly to EDP208 pili. Comparing EDP208 pilin to F, it was found that both contain the equivalent of two to three hexose units per subunit as well as blocked N-termini. EDP208 pilin contains one covalently linked phosphate residue per subunit, whereas the F pilin subunit contains two such residues. Although notable differences were found in the case of three or four amino acids, the overall amino acid compositions of F and EDP208 were very similar. Moreover, the tryptic peptide maps of the two proteins contained seven peptides with similar mobilities, suggesting considerable homology in their amino acid sequences. Substantial similarities were also noted in the secondary structures of F and EDP208 pilin on the basis of circular dichroism studies. The alpha-helix content of both proteins was calculated to be 65 to 70%. X-ray fiber diffraction studies have indicated that the arrangements of subunits in F and EDP208 pili are also similar. It was concluded that F and EDP208 pili are closely related structures.