Question

3. Describe the “logic circuit” of the Lac operon. Which proteins are bound or not to DNA in each case?

a. Glucose low, lactose low.

b. Glucose high, lactose low.

c. Glucose low, lactose high.

d. Glucose high, lactose high

Answer 1

Lac operon is a catabolic inducible operon.
It is expressed only in the presence of lactose provided that glucose is absent.

Cis-elements = Promoter and operato = Cannot be complemented
Trans-factors = Diffusible products = Ex: Repressor: Can be complemented

In the presence of lactose, lac operon cannot bind to the operator and hence transcription can occur.
However, glucose plays a major regulatory role to ensure that Lac operon is not expressed in the presence of glucose.
In the presence of glucose, the adenylate cycle is inhibited. SO, it cannot produce cAMP.
In the absence of glucose, adenylate cycle can produce cAMP which forms an activator complex with CAP and mediates the transcription.

a. Glucose low, lactose low.
Repressor bound at the operator

b. Glucose high, lactose low.
CAP not bound at its respective site.
Repressor bound at the operator

c. Glucose low, lactose high.
CAP bound at its respective site.
Repressor not bound at the operator

d. Glucose high, lactose high
CAP not bound at its respective site.
Repressor not bound at the operator