We engineered the Ferric Uptake Regulator (Fur) system to create an iron-responsive biosensor. The animations below depicts the natural Fur systems in action. The system works as follows: Fur proteins bind ferrous iron to form a Fur-Fe complex. Then this complex binds to the Fur Box promoter sequence. Once Fur-Fe is bound to the promoter, it blocks transcription of the downstream target gene. Thus, Fur represses the transcription of its target genes in response to ambient iron. In our project, we constructed a "genetic inverter" that re-engineers the Fur system to activate gene expression in response to iron.