Discussion

Iron is an essential nutrient in African trypanosomes. Bloodstream trypanosomes derive iron from host transferrin via the ESAG6/7 transferrin receptor. Iron is subsequently released from transferrin in the lysosome as FeIII. However, the pathway by which FeIII is reduced to FeII, then transported into the cytosol has not been resolved. The minimal requirement for this process would be a ferric reductase and a cation channel/transporter. We have previously shown that the cation transporter TbMLP is involved in iron transport. We have constructed bloodstream form null mutants of the two transmembrane ferric reductases (FRs) in the T. brucei genome. The FRs are orthologues of cytochrome b561 (TbCytb561) and cytochrome b558 (TbFre1), respectively. Both null mutants show no growth defect in vitro or in a murine model. The null mutants present growth phenotypes under iron-limiting conditions. TbCytb561 is also dispensable in the procyclic form with a different phenotype to the BSF. We have also constructed cell lines in which both TbCytb561and TbFre1 have been deleted following four rounds of gene deletion in the BSF. Their phenotype is more complex as the response to specific iron binding drugs differs depending on the target protein. Our current model for iron uptake will be presented.