Engineering the logical properties of a genetic AND gate

Synthetic biology promises to enhance our ability to control biological systems by creating a systematic approach for the construction of genetic circuits that reliably program cellular function. As part of this approach, efficient methods are needed for the tuning of genetic circuits so as to allow for optimization of a design despite varying cellular contexts and incomplete understanding of in vivo biological interactions. Here we outline an optimization method that we have used to improve the logical responses of a genetic AND logic gate derived from components of the LuxI-LuxR bacterial quorum-sensing system. Basing our approach on the idea of evolutionary design, we improved the properties of our genetic AND logic gate by using directed evolution and a two-step screening process to alter the activities of the LuxR transcriptional activator. Using this method, we were able to rapidly enhance the AND gate's logical responses and have increased the specificities of these responses by ∼1.5-fold.