Prochazka, L. et al. Nat. Commun. 5, 4729 (2014).

Many biological systems are based on 'bow-tie' architectures that integrate multiple inputs, synthesize them with a much smaller number of processes (the knot of the bow tie) and result in highly diverse outputs. This structure allows for an efficient use of resources and a trade-off between robustness and complexity. Prochazka et al. now apply a synthetic bow tie to mammalian cells to report on microRNA levels. In a reversible configuration, the bow tie uses the input signal of up- or downregulated microRNAs to control the expression of an activator that in turn regulates the expression of two output reporter genes. In a second configuration, the bow tie permanently records the microRNA inputs by having the activator control a recombinase that permanently switches on reporter expression. The components of bow ties are modular and can be scaled to accommodate additional inputs and outputs.