Abstract
Selection for high-producing cells in a mixed population is of great significance for synthetic biology and metabolic engineering applications. Here, we developed a cell selection mechanism that utilized a product-responsive biosensor to control the expression of E. coli endogenous toxin hipA or antitoxin hipB genes for selective removal of low-performing cells. This approach eliminates the use of exogenous antibiotics as the selection marker and offers a solution to flexibly meet the need of using either downregulating (off-switch) or upregulating (on-switch) biosensors. As a proof-of-concept, we showed that the developed cell selection systems encompassing a tryptophan biosensor (off-switch) and the toxin hipA gene dramatically enhanced the tryptophan production in E. coli, which was mechanistically characterized by monitoring the dynamic expression of the GFP-labelled hipA gene. The cell selection system was also extended for phenylalanine over-production using a phenylalanine biosensor (on-switch) and the antitoxin gene hipB. Our findings show that this approach has strong potential for wide applications in synthetic biology and metabolic engineering.