Question

The Bicoid protein binds DNA as a monomer, recognizing multiple adjacent sites within the enhancer regions of its target genes. How might the presence of these adjacent sites contribute to the sharp boundaries that exist between regions where bicoid target genes are expressed and those where they are silent?

Answer 1

Bicoid monomers are thought to interact cooperatively with one another to bind DNA and stimulate target gene expression. Cooperative binding by Bicoid to its sites in the enhancer element is thought to result in a sharp posterior boundary of hb expression that specifies the position of the cephalic furrow.
Enhancer containing multiple sites would be comparatively rare such a region would act as a local sink for Bicoid monomers. DNA-bound Bicoid interacts with only one other Bicoid monomer at a time. This binding appears to be sequential: Bicoid binds DNA as a monomer and engages in cooperative interactions only when bound to DNA.