Recombinatorial Logic

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Science  03 May 2013:
Vol. 340, Issue 6132, pp. 554-555
DOI: 10.1126/science.1237738

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Logic gates evoke images of circuit boards, but cells are arguably equally good in relying on logic computations. A classic example is the Lac operon, which activates itself upon the condition “lactose AND NOT glucose” (1). In recent years, there have been multiple reports on rationally designed, genetically encoded logic gates and circuits in living cells (2). Just like the Lac operon, these gates receive two or more molecular signals (inputs) and generate a product (output) whose level is logically linked to the inputs. Sixteen different logic connections are possible with two inputs and one output, but many of these operations have remained refractory to rational design. The trickiest of these gates usually make general statements about the inputs without referring to their exact values, such as “both inputs are the same” (an XNOR gate) or “two inputs are different” (an XOR gate). Two studies, one on page 599 of this issue by Bonnet et al. (3) and one by Siuti et al. (4), describe approaches that produce any of the 16 gates, including the notorious XNOR and XOR, in a compact manner by making relatively minor tweaks to the gates' genetic building blocks.