The lac operon controls genes involved in lactose transport and metabolism in E. coli. It contains three genes - lacZ, lacY, and lacA - that are regulated by a single promoter. The lac repressor protein binds to the operator region between the promoter and genes, inhibiting transcription in the absence of the inducer allolactose. When allolactose is present, it binds to the repressor and causes a configuration change that allows transcription of the genes and expression of beta-galactosidase, beta-galactoside permease, and beta-galactoside transacetylase. This classic model of operon regulation was discovered by François Jacob and Jacques Monod in their work

2. Lac Operon
Rani Summeya Seme 33

3. • Operon:-
An operon is a cluster of genes under the control of a
single promoter.
• Promoter:-
A promoter is a region of DNA that initiates transcription of a
particular gene.
• Operator:-
An operator is a segment of DNA regulating the activity of
genes.

4. • Lac Operon:-
It is an operon required for the transport
and metabolism of lactose.
Gene in the lac operon:-
– Lac Z encodes β-galactosidase
– Lac Y encodes Beta-galactoside permease
– Lac A encodes β-galactoside transacetylase
• Lac I Gene:-
o (Lac repressor)
o Located upstream of operon
o Inhibits the expression of genes

5. Structure of Lac Operon:-

6. Gene regulation by repressor:-
• In the absence of inducer
• Protein recognizes operator
• Configuration changes
• Transcription inhibited
• Process negative repression.

7. In the presence of inducer
• Union of allolactose with repressor
• Change the structure of repressor.
• Transcription permitted.
• Process is called positive repression.

8. It would be wasteful to produce the enzymes when
• 1. No lactose available
• 2. More preferable C source available

9. When lactose present:-

10. Development of classic model:-
• François Jacob and Jacques Monod
• Working on E. coli
• Diauxic growth
• Media having Glucose and Lactose

11. Summary:-
Glucose Lactose CAP binds Repressor binds
Level of
transcription
+ - - + No transcription
+ + - -
Low-level
transcription
- - + + No transcription
- + + -
Strong
transcription