Gene circuit engineering to improve the performance of a whole-cell lead biosensor.

To improve the performance of a whole-cell biosensor for lead detection, we designed six gene circuits by re-configuring the regulatory elements and incorporating positive feedback loops to the circuits. The lead resistance operon pbr encodes six genes with pbrRT on one side of the promoter and pbrABCD on the other side. PbrR, the divergent promoter it regulates, and GFP were used to design the lead biosensors. One has pbrR and gfp on opposite sides of the promoter mimicking the native operon. We re-configured it by placing pbrR and gfp on the same side or under two separate promoters. The one with pbrR and gfp on the same side demonstrated lead sensitivity 10 times higher than the others. Positive feedback loop was introduced to these circuits. The strength of the output signal from the designs with positive feedback loop was 1.5-2 times stronger than those without positive feedback. This study demonstrates the importance of configuration and positive feedback as effective strategies to improve the performance of lead biosensors and they can be extended to the design of other whole-cell biosensors.