Fig. 3.

Progression toward sporulation ω and probability of competence transition τ⁻¹ as a function of the sensed signals. (A) Rate of Spo0F phosphorylation as a function of the stress signal and the SpooF* levels for different levels of Spo0F dephosphorylation q (by Spo0B and RapAB). Black solid lines represent ω = 0 and dashed lines represent ω(stress) for Spo0F* = 0.5. Green dots indicate the value of stress necessary for Spo0F* to continue growing. If the level of stress falls below this value, there will be a reduction on the levels of Spo0F*. (B) Probability of competence transition τ⁻¹ as a function of quorum-sensing pheromone ComX and Rap pheromone C (CSF). Competence needs the presence of both of these inducers, and the probability increases with higher levels of ComX and CSF. If the pheromones are present in excess, the probability of competence transition reaches a limit defined by the maximum production of ComS and the noise in the system. Parameters are F = N = 1, K₁ = 1, K'₁ = 0.1 for the phosphorelay, A = 2, K_d = 1, K_p = 0.1, K_b = 5 for the quorum-sensing mechanism, maximal ComS production is S = 1.8, and the parameters for the Hill function regulating RapC and ComS production are a₀ = 1 and m = 2. Parameters are explained in SI Appendix.