Activators

Just like repressors can inhibit a gene, activators can enable it. Just like repressors, activators’ behavior can also be changed by small molecule inputs. Another example from our bacterial friends is the LuxR activator, which only acts as an activator in the presence of the compound ligand.

Here’s a diagram, with the activator labeled A:

A diagram of a gene with and without an activator. Credit: CalTech

For activators, we have a different curve (seen below in red, contrasted to the repressor curve in blue):

$$\beta(a)=\beta_0\frac{p_\text{bound}}{p_\text{tot}}=\beta_0\frac{a/K_d}{1 + a/K_d}$$

As you can see, an activator has exactly the opposite effect as a repressor.

That was a short section. Onward!