Design of a biological half adder

The building of complex systems from basic logic gates is one of the hallmarks of circuit design in electrical engineering. The question arises whether a similar strategy can be adopted for the design of artificial biological systems. In this paper, we present the design of two logic gates, a biological ANDand a biological XOR. They can be combined to produce a half-adder, one of the fundamental elements of complex systems engineering, and represent a promising basis for the design of more complex genetic circuits. Design space exploration allowed us to screen gate variants, while sensitivity analysis of refined models contributed to the specific implementation of the gates at the DNA level. The XOR gate is based on two specific proteases, which reciprocally inactivate co-synthesised transcription factors. The AND gate is designed such that, in the presence of two signals, a tRNA suppresses the premature termination of T7 RNA polymerase translation. Computer models confirmed that both designs allow gate behaviour that is reasonably close to idealised gates.