To date, some TCRSs have been identified that sense organic compounds (benzene, toluene, ethylbenzene, biphenyl, styrene, fumarate, and malate) and regulate the gene expression of proteins involved in catabolic pathways. These compounds can be metabolized and used as a carbon source for most groups of microorganisms . In TCRSs, the signal recognized by the sensor kinase domain catalyzes the ATP-dependent phosphorylation of a conserved histidine residue in the protein. The phosphoryl group is then transferred from the histidine to an aspartate residue located in the RR. The phosphorylated RR binds to specific promoter sequences to either activate or repress transcription. At present, a wide range of synthetic genetic circuits has been developed that can couple a sensor output to a desired biological activity . In addition, numerous genetic switches are also available to turn on gene expression once a target molecule has reached its activation threshold. A switch can be assembled using transcriptional repressors or activators, which allows the connection between the sensor output and regulation of the biological response . Several switch types have been developed to control the cellular response: inverter switches that produce a reciprocal response ; biphasic switches that use both negative and positive regulation and respond to small amounts of input ; toggle switches that use two repressors that cross-regulate each other’s promoters ; and riboswitches that regulate gene expression by inhibiting protein synthesis . Likewise, many logic gate types have been developed for biological circuits, including ‘NAND’, ‘NOT IF’ and ‘NOR’ .