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 Fur protein binds 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 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.