A simple genetic half adder engineered in Escherichia coli

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Richard Kil, S Bagh, DR McMillen

University of Toronto, Canada

Due to the inherently noisy environment of the cell, it is often advantageous for synthetic biology applications to use engineered genetic circuits that follow pathways with predictable logic, and whose outputs are quasi-digital “high” or “low” values. By using modular components, these devices can be combined with increasing complexity to broaden the number of functions and tasks accessible within biological systems. We describe here the design, construction, and characterization of a simple genetic half adder – one of the building blocks of arithmetic logic circuits – comprising two plasmids transformed into E. coli. Isopropyl -D-1-thiogalactopyranoside (IPTG) and doxycycline (DOX), act as input signals eliciting specific responses from the cell. Each of the input molecules is independently sensed by the first plasmid expressing enhanced green fluorescence protein as an output signal; the first plasmid behaves as an OR gate. The simultaneous presence of both input molecules is sensed by the second plasmid, responding in two ways: 1) td-Tomato is expressed as an output signal; the second plasmid behaves as an AND gate, 2) the cI lambda repressor is expressed, inhibiting the expression of the output signal for both inputs of the OR gate; the second plasmid also converts the OR gate into an XOR gate. The combination of these Boolean logic gates thus results in a half adder with robust, quasi-digital responses. Since half adders can be combined to form full adders and other more complex arithmetic circuits, the realization of this modular genetic element will hopefully expand the potential for increased control over more complex, engineered genetic networks.