Ferrioxamine transport mutants and the identification of the ferrioxamine receptor protein (FoxA) in Erwinia herbicola (Enterobacter agglomerans)

Iron deprivation of Erwinia herbicola (Enterobacter agglomerans) induces the biosynthesis of six high-Mr outer-membrane proteins and large amounts of ferrioxamine E. Mutagenesis with N-methyl-N'-nitro-N-nitrosoguanidine and selection with ferrimycin A yielded mutants of E. herbicola K4 (wild type), defective in the expression of a 76-kDa outer-membrane protein, as determined by SDS/polyacrylamide gel electrophoresis. While in bioassays wild-type cells showed growth promotion in the presence of ferrioxamines (B, D1, D2, E, G), enterobactin, citrate, ferrichrome and coprogen, these mutants failed to respond to ferrioxamines. Moreover, experiments with 55Fe-labelled siderophores confirmed that iron transport mediated by ferrioxamine E and B in the mutants was completely inhibited, whereas iron transport by Other hydroxamate siderophores, such as ferrichrome and coprogen was unaffected. The results are evidence that the 76-kDa protein in the outer membrane represents the receptor protein (FoxA) for ferrioxamines in E. herbicola.