Compositional Genetic Inverter Circuits for Complex Logic Functions

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Swati Carr, Douglas Densmore

Boston University, United States

Inverters (NOT-gates) are elementary Boolean logic circuits, where the output is absent when the input is present. Inverters mimic the inverse relationships between molecules in natural systems that are responsible for maintaining steady state, and as such form an essential tool in the synthetic biologist’s toolkit. Here we are using two separate synthetic genetic inverter circuits to build functionally complete, modular NAND-gates in a single cell by coupling them to a common output. This project explores Device-level modularity and composition, where only whole Devices are tuned, versus Part-level modularity, where most individual Parts or smaller groups of Parts are first tuned and then assembled into larger devices. The component Parts are all from publicly available repositories. We have a library of 10 distinct inverters, each with five different strengths of repression. Each inverter produces a diffusible, intercellular signalling molecule as its intermediate output, which in turn induces the final output. Both NOR and NAND-gates are functionally complete and can be used to construct all other types of combinational logic. Modular synthetic genetic NOR-gates have been previously constructed by the addition of a second input module to an existing inverter, but the additional input module/promoter introduced unexpected changes in the circuit output, requiring significant post-construction tuning. Because inverter-based NAND-gates contain only a single promoter per circuit, we expect to see fewer variations in inverter output expression requiring additional tuning. In keeping with our goals of maintaining device-level modularity, we have chosen to make NAND-gates from our inverters. We aim to show that NAND-gates, like NOR-gates, can also be constructed easily and modularly, and with less manual correction. Modular device-level composition is especially advantageous in a microfluidics framework where individual devices can be arranged to communicate with each other to make complex, scalable logic circuits.