Figure 6.
Behaviour of alternative regulatory motifs with similar structure. (a) The ‘direct feedback’ model, where levels of A directly regulate the production and degradation of A. (b) The ‘single-pathway’ model, where a single species X responds to A levels and influences both synthesis and degradation of A. (c–e) Two pathways are required for noise induction through symmetric regulation. Traces of hormone levels in five stochastic simulations, as in figure 2, for increasing Λ. The two-pathway system (c) allows increasing Λ to induce noise as above; the single-pathway (d) and direct feedback (e) systems show no increased noise with Λ. (f,g) Behaviour of hormone levels under alternative regulatory models. (f) Noise η, scaled noise η′ and logarithm of mean level ϕA as Λs and Λd vary in the ‘direct feedback’ model. Negative feedback alone can drive noise below its usual λ/ν level. (g) Noise, scaled noise and mean level for parameter changes in the ‘single-pathway’ model.