Figure 8. Excitable molecular network.

A positive feedback loop based on template ɛtoɛ is connected to the cognate drainɛ. The sequences are designed in such a way that the amplification process is also linked to a negative feedback reaction driving the consumption of triggers ɛ³. A series of replicates are initialized with different concentrations of trigger (from tubes #1 to #6: 0, 1.25, 5, 10, 20, 40 nM ɛ) and only the tube #6 shows a strong excitable response. At t=545 and t=1,435 min (blue arrows), triggers below (5 nM) or above (40 nM) the amplification threshold are injected in some of the tubes. Consistently, strong responses observed only above the threshold confirm the excitable character of the system. The inset displays the two-dimensional phase plot of the corresponding fluorescence traces, demonstrating the return to the 0 stable state after the excursion. The tube #7 contains only the template ɛtoɛ (no drain). It exhibits an oscillatory behaviour since the 0 state is unstable and cannot trap the system at the end of a cycle.