The formation of pattern by the creation and interpretation of morphogen gradients is one of the most fundamental developmental mechanisms by which organisms build themselves. There have been many attempts at understanding these mechanisms on a quantitative level but they have all been hampered by the need to study them within the context of a complex evolved organism. Very simple molecular mechanisms are capable of producing complex emergent behaviours but that simplicity is obscured by being embedded within transcriptional networks onto which the tweaks and filigrees of millions of years of evolution have accreted. We have developed a system that allows us to rebuild these developmental mechanisms in isolation in E. coli, build complete mathematical models, and parameterise those models against data to make quantitative predictions about complex multicellular behaviours. We have engineered an optimized double receiver circuit that orthogonally responds to 3O-C6-HSL or 3O-C12-HSL quorum sensing molecules with different outputs and used this receiver as the basis for a number of patterning circuits. By engineering mutual inhibition between the two signalling systems, we have built a circuit that responds to one signal or the other but not both. This double exclusive reporter recapitulates the mutual inhibition mechanism that is proposed to be central to producing mutually exclusive domains of gene expression with sharp boundaries in the early Drosophila embryo and the mammalian neural tube. By studying this mechanism in isolation we show that such a topology can indeed be bistable and, due to hysteresis, produce stable boundaries of gene expression despite changing morphogen gradients. In addition, by adding signal production as well as mutually exclusive reception, we can create two domains of gene expression with a self-organizing boundary in response to a single morphogen gradient.
Pázmány Péter Catholic University, Esztergom, Hungary