Iron is essential for most organisms. However, two problems are associated with the use of iron for aerobically growing organisms: (i) its accumulation leads to the formation of toxic reactive oxygen species and  (ii) it is present mainly as the highly insoluble ferric iron which makes the access to iron difficult. As a consequence, a tight regulation of iron homeostasis is required. This regulation is achieved in many bacteria by the ferric uptake repressor Fur. The way how the activity of Fur is controlled, has so far remained elusive. Here, we have identified the Fur antirepressor FurA (previously YlaN) in the model bacterium Bacillus subtilis and describe its function to release Fur from the DNA under conditions of iron limitation. The FurA protein physically interacts with Fur, and this interaction prevents Fur from binding to its target sites due to a complete re-orientation of the protein. Both in vivo and in vitro experiments using a reporter fusion and Fur-DNA binding assays, respectively, demonstrate that the Fur-FurA interaction prevents Fur from binding DNA and thus from repressing the genes required for iron uptake. Accordingly, the lack of FurA results in the inability of the cell to express the genes for iron uptake under iron-limiting conditions. This explains why the furA gene was identified as being essential under standard growth conditions in B. subtilis. Phylogenetic analysis suggests that the control of Fur activity by the antirepressor FurA is confined to, but very widespread in bacteria of the class Bacilli.

Control of iron homeostasis by a regulatory protein-protein interaction in Bacillus subtilis: The FurA (YlaN) acts as an antirepressor to the ferric uptake regulator Fur