Fig. 1. Biosensor design. Natural and engineered chimeric promoter–operators (P_ferB, P_PC, and P_LC) were inserted upstream from the eGFP reporter gene. FerC binds to the promoter–operator sequence(s) repressing expression of the reporter gene. In the presence of FerA, ferulic acid (FA) is converted into feruloyl-CoA (FA-CoA), which in turn de-represses FerC and activates gene expression.