Iron is essential, but must be handled carefully due to its toxicity. Acquisition of iron is problematic as it is poorly soluble. Indeed, iron is considered the most limiting nutrient in biology. This scarcity is exploited by host defences where an iron-withdrawal strategy is used as part of innate resistance against disease. Thus, pathogens must employ iron transporters that target host-iron sources. Such host-targeting systems are virulence factors and their loss results in attenuation of the pathogen. Because the battle for iron is such a key component of infectious disease outcome, much research has focussed on bacterial iron-acquisition systems revealing several types. In Gram-negative bacteria, the most common are the TonB-dependent OM receptor systems that allow uptake of iron from various ferric complexes (e.g. ferric-siderophores, haem or ferric-transferrin) often important for infection. In addition, FeoAB imports free ferrous iron under anaerobic conditions and is important for gut colonisation. Recently, we discovered a new type of iron transporter that appears widespread in bacteria, called EfeUOB. Little is known about EfeUOB and so it offers much potential for rewarding research. The system works aerobically and anaerobically, but shows preference for ferrous iron under acid conditions. Appropriately, it is induced by a combination of acidity and low iron. It consists of three components, all of which are functionally essential: EfeU, a polytopic inner membrane protein acting as a ferric permease; EfeO, a periplasmic ferrous iron binding protein that oxidises ferrous iron; and EfeB, a haem-peroxidase which may assist EfeO in ferrous oxidation.
The EfeUOB system is cryptic in some bacteria (e.g. E. coli K-12) but functional in many others (e.g. E. coli O157). This is caused by an insertion/subtraction that generates a frameshift. We wish to study the environmental factors that influence conversion to/from the cryptic state. We also wish to determine whether EfeUOB contributes to pathogenicity and/or colonisation of E. coli O157 and other host-associated (or environmental) bacteria. We also would like to how EfeUOB is distinct in the iron-uptake capacity it affords with respect to other iron transporters (often, ten or more in one bacterium). A major interest is the mechanism of transport utilised by EfeUOB, as its make up is so different from any other type of transporter. EfeO contains two domains, a small N-terminal cupredoxin (Cu-containing electron transfer proteins) and a larger C-terminal metalloprotease-75 domain. We wish to determine the structures and functions of both. We have already resolved the EfeB structure, and now wish to study its precise role in transport. It seems likely that EfeUOB uses a novel energisation process involving ferrous oxidation and so we will examine this aspect also. The specificity of the system for different metals will be studied, and evidence for ferrous oxidation by EfeUOB in vivo will be sought. The activities and functions of EfeUOB variant systems from other bacteria (e.g. Pseudomonas, Burkholderia, O157) are also being investigated or are under consideration for study (e.g. Brucella).
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