(a) Cartoon representation of a system with two hybrid HKs that share the same Hpt. (b–d) Implementation of different signal processing functions using the system shown in (a); an ‘adder’ as seen experimentally in V. harveyi [36] (b), a Boolean AND gate (c) and a Boolean OR gate (d). Each panel shows the fraction of phosphorylated RR at steady state for different auto-phosphorylation rate constants (i.e. input levels) acting on the two HKs. It is assumed that the two signals are specific for the two HKs (k1,1, k1,3 and k2,1, k2,3, respectively) and the ratios k1,1/k1,3 and k2,1/k2,3 are held fixed. The systems shown in (c,d) are bistable, with the blank regions of the surfaces corresponding to unstable regions. For the parameters used, see the electronic supplementary material, table S1.