Objective VII

What is an operon?

An operon is a cluster of functionally related genes regulated together by a single promoter and transcribed into a single mRNA. It allows coordinated expression of genes involved in the same pathway.

What is the structure of the Lac operon?

The Lac operon includes:

• A promoter (where RNA polymerase binds)

• An operator (where the Lac repressor binds)

• Three structural genes (lacZ, lacY, lacA)

What does the lacZ gene code for and what is its function?

lacZ encodes β-galactosidase, an enzyme that hydrolyzes lactose into glucose and galactose.

What does the lacY gene code for and what is its function?

lacY encodes lactose permease, a membrane protein that transports lactose into the bacterial cell.

What does the lacA gene code for and what is its function?

lacA encodes thiogalactoside transacetylase, which detoxifies harmful compounds that enter the cell along with lactose.

How does the Lac operon function in the presence of glucose and absence of lactose?

The operon is repressed:

• Lac repressor remains bound to the operator.

• Low cAMP due to glucose prevents CRP from activating transcription.

• Transcription is off.

How does the Lac operon function in the absence of glucose and presence of lactose?

The operon is activated:

• Lactose (as allolactose) binds the repressor, detaching it from the operator.

• High cAMP binds to CRP, which activates transcription.

• RNA polymerase transcribes the operon efficiently.

How does the Lac operon function in the presence of both glucose and lactose?

The operon is weakly expressed:

• Lactose removes the repressor.

• But glucose causes low cAMP, so CRP doesn’t activate transcription.

• Result: minimal transcription (basal level).

What is catabolite repression?

A form of global gene regulation in which the presence of glucose inhibits the transcription of other operons (like lac) by lowering cAMP levels.

What is the cAMP Receptor Protein (CRP) and its function?

CRP is a transcriptional activator. When bound to cAMP, it binds upstream of the Lac promoter and enhances RNA polymerase binding, boosting transcription only when glucose is low.