Chapter 17, Problem 29ESP

Although a single activator may bind many enhancers in the genome to control several target genes, in many cases, the enhancers have some sequence conservation but are not all identical. Keeping this in mind, consider the following hypothetical example:

• Undifferentiated cells adopt different fates depending on the concentration of activator protein, Act1.
• A high concentration of Act1 leads to cell fate 1, an intermediate level leads to cell fate 2, and low levels to cell fate 3.
• Research shows that Act1 regulates the expression of three different target genes (A, B, and C) with each having an enhancer recognized by Act1 but a slightly different sequence that alters the affinity of Act1 for the enhancer. Act1 has a high affinity for binding the enhancer for gene A, a low affinity for the gene B enhancer, and an intermediate affinity for the gene C enhancer.

From these data, speculate on how Act1 concentrations can specify different cell fates through these three target genes? Furthermore, which target genes specify which fates?