We engineered the Ferric Uptake Regulator (Fur) system to create an iron-responsive biosensor. The animations below demonstrate how the natural Fur systems works. The Fur protein binds iron to form a complex that binds to the Fur Box promoter sequence. Once Fur 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.