FIGURE 1 

Models of regulation by TFs. (A) TFs (rounded boxes) bind to DNA and interact with components of the transcriptional apparatus (ovals). Ultrasensitivity in the response to the TF can arise through cooperative binding of the TF to multiple sites mediated by direct contact (TF1), simultaneous contact with the same target complex (TF2), or by collaborative exclusion of nucleosomes (TF3). Binding of a monomeric TF (red) to a single site cannot provide ultrasensitivity. (B) A TF affects promoter activity by binding to DNA and recruiting a histone-modifying enzyme (squares) that alters the modification state of the local nucleosomes (circles). If modified nucleosomes themselves recruit enzymes that foster the same modification on other nucleosomes (orange arrows), in a positive feedback loop (Dodd et al, 2007), then a TF that binds non-cooperatively can produce an ultrasensitive response.