Molecular recognition of siderophores: A study with cloned ferrioxamine receptors (FoxA) from Erwinia herbicola and Yersinia enterocolitica

The outer membrane receptor for ferrioxamines (FoxA(Erw)) of Erwinia herbicola (Pantoea agglomerans) was cloned from a cosmid gene bank and partially sequenced. A comparison of the partial amino acid sequence of FoxA(Erw) with the amino acid sequence of FoxA(Yer) from Yersinia enterocolitica revealed a high sequence homology. A functional analysis of FoxA(Erw) and FoxA(Yer) receptors cloned into a Fhu-negative background (HK97) revealed that ferrioxamines are recognized at very low concentrations (<10 pmoles) in growth promotion bioassays. A collection of ferrioxamine derivatives containing varying chain lengths and ether bridges within the molecule was also accepted. However, the three ether containing ferrioxamine (Et-3) behaved differently in the two FoxA receptors. Coprogen was also recognized to a certain extent, whereas ferrichromes were completely excluded from the FoxA receptors, confirming that coprogens share some structural similarities with the ferrioxamines. FoxA mutants (FM13) of Erwinia herbicola obtained by ferrimycin selection showed no uptake of Fe-55-labelled ferrioxamine E and B any more, while the transport of coprogen and ferrichrome was unaffected or even slightly increased.