The document describes the lac operon system in E. coli for lactose catabolism. It explains that when glucose is absent and lactose is present, allolactose acts as an inducer by binding to the lac repressor and allowing RNA polymerase to transcribe the lac genes encoding β-galactosidase and other proteins for lactose metabolism. However, when glucose is present, even if lactose is also present, transcription is repressed due to low cAMP levels inhibiting the CAP-cAMP complex from activating transcription.

1. Lac Operon System
(Catabolite Repression)

2. Lac Operon System
(Turn on & Turn off)
P Lac I Lac I P O Lac Z Lac Y Lac A
Inhibitor
Lactose Allolactose
Lac Inhibitor Promoter
Lactose Catabolism
β-
Galactosidase
Permease
Transacetylase
Polycistronic
RNA
polymerase
Transcription

3. P Lac I Lac I P O Lac Z Lac Y Lac A
Lactose Catabolism
β-
Galactosidase
Permease
Transacetylase
Transcription
When Glucose is absent but Lactose is present
CAP Binding site
CAP
cAMP
CAP cAMP complex
Acts as inducer
RNA
polymerase
Glucose ∝ 1/cAMP
(Concentration)

4. P Lac I Lac I P O Lac Z Lac Y Lac A
Transcription
When Glucose is present and
Lactose is present
CAP
cAMP
CAP cAMP complex
RNA
polymerase
High glucose = Low cAMP (Concentration)

5. Four Situation is possible in
Catabolite Repression
 When Glucose is present but Lactose is absent E. coli does not produce β-Galactosidase.
 When Glucose is present and Lactose is Present E. coli does not produce(or produce a very
little amount of) β-Galactosidase.
 When Glucose is absent and Lactose is absent E. coli does not produce β-Galactosidase.
 When Glucose is absent but Lactose is present E. coli does produce β-Galactosidase.

6. Thank You