Delineating the Mechanisms of Quorum Sensing Cross-Talks By Coupling Activation and Repression

Many Gram-negative bacterial species use acyl homoserine lactone (AHL)-mediated quorum sensing to detect local population density or diffusion rates and change gene expressions accordingly. In synthetic biology, components of quorum sensing systems are often adopted to build synthetic circuits that require cell to cell communication using diffusible signals. Because different species of bacteria synthesize different AHLs and their corresponding sensor/responder proteins, quorum sensing is commonly referred to as a species-specific communication method bacteria adopted. Ironically, it is also well noted that there are considerable cross-talks between AHLs and their non-cognate receptors, making the extent of species-specificity questionable. In synthetic circuits, cross-talks between non-cognate AHLs and receptors can disrupt desired functions that are based on quorum sensing components, especially when they were employed to mimic species-specific communication effect. Such observation led to studies that examined signal cross-talks and promoter cross-talks by using GFP reporters that are activated when an AHL/receptor complex activates transcription. While these studies provided valuable information on the presence of activating cross-talks, there has been a lack of a systematic approach to easily detect and quantify the nature of repressive cross-talks. Here, we report the synthetic reporters we built to test both activating and repressive cross-talks across 10 quorum sensing systems. Our study allows us to further delineate the mechanism behind cross-talks, by distinguishing the following three aspects: AHL binding to the receptor, AHL/receptor complex binding to the DNA, and the viable interaction with the RNA polymerase. Finally, our reporter design also features a Memory element, allowing us to test the presence or the absence of AHLs in the mouse gut using engineered AHL sensor E.coli strains.