Monitoring arsenic using genetically encoded biosensors in vitro: The role of evolved regulatory genes.

Toxic pollutant (TP) detection in situ using analytical instruments or whole-cell biosensors is inconvenient. Designing and developing genetically coded biosensors in vitro for real-world TP detection is a promising alternative. However, because the bioactivity and stability of some key biomolecules are weakened in vitro, the response and regulation of reporter protein become difficult. Here, we established a genetically encoded biosensor in vitro with an arsenical resistance operon repressor (ArsR) and GFP reporter gene. Given that the wildtype ArsR did not respond to arsenic and activate GFP expression in vitro, we found, after screening, an evolved ArsR mutant ep3 could respond to arsenic and exhibited an approximately 3.4-fold fluorescence increase. Arsenic induced expression of both wildtype ArsR and ep3 mutant in vitro, however, only ep3 mutant regulated the expression of reporter gene. Furthermore, the effects of cell extracts, temperature, pH, incubation, and equilibrium time were investigated, and the equilibration of reaction mixtures for 30 min at 37 °C was found to be essential for in vitro arsenic detection prior to treatment with arsenic. Based on our data, we established a standard procedure for arsenic detection in vitro. Our results will facilitate the practical application of genetically encoded biosensors in TP monitoring.