Quorum sensing has been identified in many bacteria to offer cells unique capacity of cell density-dependent response. With this cell-cell communication mechanism, a population of bacterial cells is able to respond to environmental cues as a whole to exhibit coordinated and synchronized behavior. We are interested in engineering and applications of bacterial quorum sensing. In many applications, engineered quorum-sensing systems with altered responses are desired. In this presentation, we will discuss our work in engineering and applications or quorum sensing in synthetic biology and protein production. We have developed efficient genetic screen and selection methods to engineer quorum sensing components using directed protein evolution. In particular, we have engineered LuxR for increased sensitivity to signaling molecule concentrations. We have also enhanced the activity of LuxI to produce the signaling molecule more efficiently. In addition, the substrate specificity of RhlI has been changed dramatically. Engineered quorum sensing promoters have also been developed for reduced basal expression levels. With these engineered components, we have demonstrated that directed evolution is a useful strategy to improve outputs of artificial genetic circuits, such as genetic positive feedback loop and genetic AND gate. Because quorum sensing enables autonomous activation of gene expression, we have developed an array of engineered quorum-sensing systems which each can be activated at a unique cell density as a gene expression regulatory method. We further demonstrated that these engineered systems could be used to regulate production of proteins, a method particularly useful for production of toxic proteins in large-scale fermentation.

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