Synthetic biology has begun producing engineered biological systems with applications from therapeutics to green chemistry. Historically, a concern with genetically modified organisms has been their potential for environmental escape and spread. To address this issue, we are engineering "suicide circuits" to induce cell death under specific conditions. In particular, we have constructed an ePop circuit that can “pop” cells at high population density, by using quorum sensing genes luxR and luxI from vibrio fisheri to control expression of a lysis gene (E) from bacteriophage ΦX174. When present in MC4100z1 cells, the ePop circuit caused regular, reproducible oscillations in population densities lasting up to 5-6 complete cycles with a period of 7-10 hours in batch cultures. Furthermore, microcolonies of circuit bearing cells grown on agar showed synchronized lysis. These observations suggested that the cell lysis was controlled by density sensing. However, we still observed significant oscillations even after removing the luxI gene, which presumably should have abrogated communication and activation of E transcription. These unexpected results highlight potential interactions between synthetic gene circuits and their host strains which can contribute to system dynamics, but are often overlooked and poorly understood. In particular, we hypothesize that intrinsic regulation of slyD, a peptidyl-prolyl isomerase required for E gene folding and function plays a role in determining the timing and severity of cell lysis.

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