Fig. 1 Construction strategy of aTF biosensor and its application in food safety A typical system of aTF biosensor includes a molecular recognition element-aTF, a signal amplification strategy, a signal output system and a sensing system. The constructed aTF biosensor can be applied to detect heavy metals, pesticide and veterinary drug residues, food additives and pathogens in food and other food safety fields

Fig. 2 Schematic diagram of the mechanism of metamorphic regulation of aTF and structure of aTF A: Repressed aTF regulatory mechanism. B: Activated aTF regulatory mechanism. C: Stereoview of the overall structure of mercury（II）-bound Tn501 MerR（PDB code 5CRL）[31]. D: Ribbon plot of the TetR dimer N82A in complex with doxycycline-Mg2+（PDB code 6RGX）[25]. E: Ribbon diagram showing the overall structure of the LasR-N-3-oxo-dodecanoyl homoserine lactone homodimer（PDB code 3Ⅸ3）with one monomer colored in light blue and the second colored in purple. The homoserine lactone is shown in a ball-and-stick representation and secondary structure elements are noted on the light-blue monomer[26]

Fig. 3 Application of aTF-based in vivo biosensors in food safety A: Schematic of a biosensor based on aTF coupled with RCA for the detection of small molecules[33]. B: Schematic diagram of a biosensor based on aTF coupled to CRISPR-Cas12a for the detection of small molecules[36]. C: Schematic representation of the RNA Output Sensors Activated by Ligand Induction（ROSALIND）is used to detect contaminants in water in a cell-free in vivo transcription system[39]. D: Schematic of the colorimetric paper-based biosensor for the detection of antibiotics inhibiting bacterial protein synthesis[43]. E: Schematic representation of a smartphone-based aTF bioluminescent sensor for quantification of tetracycline and erythromycin in milk[11]. F: Schematic of a TetO/TetR-based biosensor to detect tetracycline using a glucometer[47]