Interconnection of quantitatively characterized genetic devices to engineer predictable biological functions

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Lorenzo Pasotti, Nicolo' Politi, Susanna Zucca, Michela Casanova, Maria Gabriella Cusella De Angelis, Paolo Magni

University of Pavia, Italy

Modularity is one of the hallmarks in the engineering world, as it enables the composition of systems with predictable behaviour upon interconnection of a set of quantitatively characterized components. Synthetic Biology aims to construct novel biological functions by following a bottom-up design approach. Although standard measurement approaches were proposed to facilitate the characterization of parts and the sharing of the resulting quantitative information via datasheets, real modularity is still a major issue and a number of factors may impair the predictability of the designed system (e.g. intrinsic biological noise and cell overburdening). In order to exploit the whole potential of Synthetic Biology in the bottom-up construction of biological circuits, its modularity boundaries must be discovered. Here, model systems were used to test the modularity of different biological input devices when interconnected upstream of a fixed output device in a genetic circuit incorporated in Escherichia coli. If modularity persists, when the input modules produce identical signals the output device should produce identical signals, even if the input modules are structurally different. Three input systems were used: 1) a set of constitutive promoters of different strengths, 2) an IPTG-inducible promoter and 3) an HSL-inducible device. These modules provide a transcriptional signal that drives the output device, which is a tetR-based logic inverter. First, input modules were individually characterized via RFP. Then, they were assembled upstream of the inverter and GFP was used to characterize the output of the interconnected systems. The resulting static characteristics were analyzed to experimentally evaluate the modularity of the used components. The logic inverter was re-used to construct a cold-inducible system by interconnecting a pre-characterized heat-inducible device upstream. The transfer functions of the devices well-predicted the experimental output of the constructed circuit. Temperature-dependent behaviour of the used components was tested and dynamic performance was investigated.