Fig. 2.

Six-state model of a specific site (e.g. a promoter) that is active if two cofactors are bound to the specific target site (red boxes). The unspecific site (white boxes) and the specific target site can contain two monomers at the same time. The equilibrium constants are written in terms of the ratios bind/unbind with the rates K_l= λ_l/μ_l l = 1,2. The direct arrival at the target site with 3D diffusion of the protein is K = λ/μ. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)