Characterization of the ferrioxamine uptake system of Nitrosomonas europaea

The chemolithoautotroph Nitrosomonas europaea has two genes predicted to encode outer-membrane (OM) ferrioxamine transporters. Expression of the ferrioxamine uptake system required induction, as shown by the shorter lag phase in ferrioxamine-containing cultures when ferrioxamine-exposed cells were used as an inoculum. The two OM ferrioxamine siderophore transporters encoded by foxA₁ (NE1097) and foxA₂ (NE1088) were produced only in cells grown in Fe-limited ferrioxamine-containing medium. The inactivation of foxA₁, singly or in combination with foxA₂, prevented growth in Fe-limited medium containing excess desferrioxamine (DFX). The foxA₂-disrupted single mutant grew poorly in the regular Fe-limited (0.2 µM) medium with 10 µM DFX, but grew well when the Fe level was raised to 1.0 µM with 10 µM DFX. For efficient acquisition of Fe-loaded ferrioxamine, N. europaea needs both ferrioxamine transporters FoxA₁ and FoxA₂. FoxA₁ probably regulates its own production, and it controls the production of FoxA₂ as well.