A Synthetic Bistable System in Yeast

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Robert Gnuegge, Moritz Lang, Fabian Rudolf, Joerg Stelling

ETH Zurich, Switzerland

Bistable systems are ubiquitous in nature. Prototypical examples are the S. cerevisiae GAL network and the E. coli lac operon. The bistable and hysteretic behavior of such systems has been analyzed in depth in theoretical and experimental work, but important open questions remain and conflicting results have been reported. For instance, it is unclear how different feedback loops impact on the bistable behavior and a detailed description of how noise influences the switching of cells between different stable states is lacking. Quantitative experimental analyses performed so far have tried to answer these questions for systems in their natural context. However, in this situation the system behavior might be influenced by interactions with unknown cellular factors. To overcome these limitations we implemented a synthetic bistable system resembling the E. coli lac operon in S. cerevisiae based exclusively on orthogonal parts. To determine the state of the system in individual cells we implemented fluorescent protein based reporter systems. Quantitative analysis of the system’s behavior by flow cytometry and fluorescence microscopy demonstrated the system’s bistable and hysteretic properties. We developed a dynamic mathematical model that can reproduce the experimental findings. It predicted how to modify the system to expand the bistable range and to influence switching rates. We demonstrate how the combination of quantitative analysis and mathematical description helps to design bistable synthetic systems in yeast. The system presented here is more complex than positive feedback circuits that were previously established in yeast. Such increased complexity will expand the scope of synthetic systems to understand natural systems.