The innate immune defense systems respond to bacterial infection by reducing the amount of available iron in order to restrict bacterial growth. Pathogens employ host-specific iron uptake systems to overcome this host iron-withdrawal response. Most cellular organisms (bacteria as well as host cells) can also store iron intracellularly and use this stored iron to counter iron restriction. Cellular iron stores are located in proteins called ‘ferritins’ – such proteins are universally present in living organisms. However, it remains unclear how iron stores are released from ferritins. Our work shows that a novel protein, YqjH (a flavo-ferric reductase), from E. coli performs this role by reduction of free flavins which then act as electron delivery agents to reduce and thus mobilise the iron within ferritin iron cores. We now wish to study this effect further to provide a more complete understanding of its action. There are several known or potential iron stores in bacteria such as E. coli (FtnA, FtnB, Bfr, Dps, CyaC, Fe-S proteins) and we wish to determine the role of YqjH in accessing all potential/known iron stores. We also wish to determine whether YqjH works heterologously (i.e. on iron storage proteins from other organisms) to test the specificity of the interaction. The possibility that YqjH may cause toxicity (due to excessive iron release) if expressed under high iron conditions will be tested. The yqjH gene is repressed by iron (Fur dependent) and is also controlled by the repressor YqjI under low Ni2+ conditions.
Other proteins also appear to contribute to iron recycling (FhuF and Bfd, both Fe-S proteins) – the contribution and specificities of these proteins will also be studied in vivo and in vitro. We will also look at the roles of other flavin reducing enzymes (e.g. Fre) in iron recylcling. In addition, we have identified an extracellular ferric reductase (cytochrome b561 homologue 2 or YceJ) that converts external ferric iron to the more soluble ferrous form. The activity of this reductase is lost when siderophore production is disabled indicating that the reductase activity depends on the presence of siderophore. Siderophores are produced and exported by bacteria under low iron conditions and act as ferric chelators. They solubilize ferric iron and deliver it to the cell for uptake and incorporation. They are key components of the iron uptake machinery. The reason for the dependence of YceJ ferric reductase activity on siderophore is unclear and so this will be studied. Possibly, the siderophore delivers ferric iron to the reductase (which resides in the inner membrane) or the siderophore acts as a conduit for delivery of electrons to extracellular ferric iron. Both hypotheses will be tested.
Roles of these systems in gut colonisation will also be explored using appropriate models. We will particularly examine the role of the Efr system in the release of iron sources found within the diet. Since the human host employs an extracellular ferric-iron reduction system within the gut to obtain iron from the diet, the possibly role of a similar bacterial Efr system in mobilising iron for uptake by the human host will be tested. The role of the gut microbiota in mobilisation of dietary iron has been little studied to date. This aspect is directly related to the food security priority area of the University.
Another area of study will be the role of the haem group of bacterioferritins (Bfr) in mediating release of iron from the protein. We will replace the natural Bfr with a haem-free variant, and then test for the capacity of the haem-free variant to act effectively in iron storage and release. We will also examine the role of the Bfr-associated Bfd protein in iron release from the haem-free and haem containing forms of Bfr.
The PhD programme will allow the applicant to experience a wide range of important molecular techniques (including PCR, bacterial genetics, protein over-expression, directed mutagenesis, cloning) and will provide much scope for independence, publication and attendance at scientific conferences. The work is novel and exciting, and would be expected to lead to high impact outputs. The lab is well equipped and the candidate would join a well-established group working on related projects that currently includes seven PhD students, one post-doctoral Fellow and one research technician.
High quality International students with funding are strongly encouraged to apply. Many other related/unrelated projects are also available and details can be provided upon request. UK/EU students with an interest in this project should apply in good time so that funding sources can be identified and funding sought
Andrews, S.C., Robinson, A.K. & Rodriguez-Quinones, F., (2003). Bacterial iron homeostasis, FEMS Microbiology Reviews, 27, 215-237.
Cornelis, P., Wei, Q., Andrews, S.C. and Vinckx, T. (2011) Iron homeostasis and management of oxidative stress response in bacteria. Metallomics 3, 540-549.
Bamford, V.A., Armour, M., Mitchell, S.A., Cartron, M., Andrews, S.C. & Watson, K.A., (2008). Preliminary X-ray diffraction analysis of YqjH from Escherichia coli: a putative cytoplasmic ferri-siderophore reductase, Acta Crystallographica, F64, 792–796.
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