Excitations occur when there is cleavage of mRNA by the toxins.

A: Scheme of the minimal system that we use to explain toxin excitations. DNA is transcribed to mRNA which is translated to toxins T and antitoxins A. These can combine to form the complexes AT. If the toxin level reaches a threshold this will cause cleavage of the mRNA. B: Gillespie simulation of the minimal system. In stress conditions (highlighted region) the system is only excitable when mRNA cleavage is included. C: Scheme of the system when including secondary complex (TAT) formation and DNA binding of complex AT, inhibiting the mRNA transcription. D: Gillespie simulation of the system with TAT formation and DNA binding by AT, with and without mRNA cleavage. E: Scheme of the entire system, including translational inhibition as well as inhibition of the growth rate when the toxin level is increased. F: Gillespie simulation of the entire system, with and without mRNA cleavage. Without mRNA cleavage there are increased toxin levels, but pronounced excitory behavior is observed when including mRNA cleavage. For the details of the Gillespie simulations, see supporting information S1 File. Parameters: see Table 1. Additional in (B): c1 = 1.28 × 10−5 s−1, c2 = 0.00231s−1, in (C): st = sm = 1, bt = bm = 0.027. In the highlighted regions the stress is increased with a factor two by using da = 0.00462s−1.