IDENTIFICATION OF A NEW SITE FOR FERRICHROME TRANSPORT BY COMPARISON OF THE FHUA PROTEINS OF ESCHERICHIA-COLI, SALMONELLA-PARATYPHI-B, SALMONELLA-TYPHIMURIUM, AND PANTOEA-AGGLOMERANS

The fhuA genes of Salmonella paratyphi B, Salmonella typhimurium, and Pantoea agglomerans were sequenced and compared with the known fhuA se quence of Escherichia coli, The highly similar FhuA proteins displayed the largest difference in the predicted gating loop, which in E, coli controls the permeability of the FhuA channel and serves as the princ ipal binding site for the phages T1, T5, and phi 80. All the FhuA prot eins contained the region in the gating loops required in E. coli for ferrichrome and albomycin transport. The three subdomains required for phage binding were contained in the gating loop of S, paratyphi B whi ch is infected by the E, coli phages, whereas two of the subdomains we re deleted in S. typhimurium and P, agglomerans which are resistant to the E. coli phages, Small deletions in a surface loop adjacent to the gating loop, residues 236 to 243 and 236 to 248, inactivated E, coli FhuA with regard to transport of ferrichrome and albomycin, but sensit ivity to T1 and T5 was fully retained and sensitivity to phi 80 and co licin M was reduced 10-fold. Full-size FhuA hybrid proteins of S. para typhi B and S. typhimurium displayed S. paratyphi B FhuA activity when the hybrids contained two-thirds of either the N- or the C-terminal p ortions of S, paratyphi B and displayed S, typhimurium FhuA activity t o phage ES18 when the hybrid contained two-thirds of the N-terminal re gion of the S, typhimurium FhuA, The central segment of the S. paratyp hi B FhuA flanked on both sides by S, typhimurium FhuA regions conferr ed full sensitivity only to phage T5. The data support the essential r ole of the gating loop for the transport of ferrichrome and albomycin, identified an additional loop for ferrichrome and albomycin uptake, a nd suggest that several segments and their proper conformation, determ ined by the entire FhuA protein, contribute to the multiple FhuA activ ities.