Siuti, P. et al. Nat. Biotechnol. advance online publication (10 February 2013).

One of the central goals in synthetic biology is to develop gene circuits that can be programmed to replicate cellular networks. Desired features in such circuits are logic gates that can easily be programmed and maintain memory so that the cellular states produced by the logic gates are retained over time. Siuti et al. used two inducible recombinases, a promoter and a terminator flanked by recognition sites for the recombinases and GFP as an output signal to create 16 Boolean logic functions. They arranged the regulatory elements in the desired orientation for a given logic gate using Gibson assembly and showed that cells maintained stable output memory for many generations after the input was withdrawn. As an application for a higher-order network the researchers built a digital-to-analog converter that allows cells to translate a digital input signal, activating the recombinases, to stable analog gene expression output.