Fig 1 Diagram of a genetic iron sensor. Iron binds the Ferric Uptake Regulator (Fur) to form a complex with high affinity for the Fur box in the promoter, here shown as the aceB promoter. Once the iron-Fur complex is bound to the promoter, it represses transcription of the target gene GFP. GFP expression is thus negatively correlated with iron availability.

Fig 2 Construction of an iron-responsive genetic element by fusing a Fur-regulated promoter with a reporter gene. Promoter-reporter fusions were made with flanking restriction sites that are compatible with Biobrick-based cloning.

Table I Genetic elements used to make iron-responsive sensors.

NAME	FIGURE	DESCRIPTION
E. coli promoter with Fur binding site		iron-Fur complex binds promoter to repress expression
sfGFP		Fluorescent reporter gene
Terminator		terminator to stop transcription
Plasmid		Biobrick-compatible plasmid backbone