Stereoselective recognition of ferrichrome by fungi and bacteria

Ferrichrome is an iron chelate of the hydroxamate type, originally isolated from the culture fluid of the smut fungus Ustilago sphaerogena [1]. It was later shown that ferrichrome is also widespread among strains of Aspergillus and Penicillium [2]. Although ferrichrome is a fungal siderophore, synthesized to transport iron into fungal cells, it is also rapidly taken up by enteric bacteria, such as Escherichia coli [3,4] and Salmonella [5]. Ferrichrome consists of three glycyl and three 6-N-acetyl4J-N-hydroxy-L-ornithinyl residues [6,7]. The trivalent iron is octahedraly coordinated by the three hydroxamate groups yielding a A-cis coordination about the iron atom [8,9]. The absolute configuration of ferrichrome has recently been determined by X-ray diffraction [10]. Earlier results from our laboratory [11] gave evidence that the fungus Neurospora crassa discriminated between ferrichrome and the synthetic enantio-ferrichrome [ 12]. However, ferrichrome was not a product of this fungus and its transport rate was very low compared to that of coprogen, the strain specific siderophore [13]. We therefore investigated the transport of ferrichrome and enantio-ferrichrome (Fig. 1) into a ferrichrome producing fungus. We selected Penicillium parvum which synthesized exclusively deferri-ferrichrome in response to iron limitation. Furthermore, it was of interest to compare the stereoselective recognition of the siderophores in P. parvum with that in E. coli. The results of the present investigation indicate that ferrichrome but not enantio-ferrichrome was recognized as a siderophore in P. parvum. However, enantio-ferrichrome was taken up by E. coli, although with a slower rate (approx. 50%) than ferrichrome, suggesting that the recognition of ferrichrome by E. coli is less selective than by P. parvum.