Figure 1.

Schematic of the population-genetic model. A schematic cartoon of our population-genetic model. (top) When cooperativity is absent different transcription factors (gray and red) must bind to sites at each of their targets independently. Each factor has a number of targets, K₁and K₂, and a number β(K1 + K2) of shared targets (bottom). When cooperativity is present, a physical interaction between transcription factors (blue line) can mitigate the need to bind independently at shared targets, but may cause misregulation at targets that are not shared, by causing the factor with which it interacts cooperatievly to misbind. Cooperatively is therefore advantageous between transcription factors that share many targets, but it may be deleterious at targets that are not shared.