Microbial Diversity

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