An operon is a functional piece of DNA in genetics that houses a collection of genes that are all regulated by the same promoter.

1. Rithika. R. S,
I – M.Sc. Bioinformatics,
Department of Biosciences,
SKASC.

2. OPERON
 Operon is a cluster of genes which is controlled
by single promoter.
 Proposed by French microbiologists Francois
Jacob and Jacques Monod in 1960.
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3. WHY OPERONS…
 Protein synthesis is controlled according to the
need.
 Provides signal to the living system when and
where to produce proteins.
 Helps to conserve energy.
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4. lacI P O lacZ lacY lacA
lacIp
β-galactosidase
Permease
Trans
acetylase
Repressor
Structure of Lac operon
Structural genes
Regulatory region
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5. Structure
◦ lac I – regulatory gene(repressor protein)
◦ Lac promoter - RNA polymerase binds to the promoter.
◦ Lac operator – repressor protein binds to the operator.
◦ Lac z– β-galactosidase, cleaves lactose into glucose and
galactose
◦ Lac y – β-galactoside permease, allows the cell to import
lactose.
◦ Lac a – Transacetylase, transfers acetyl group from acetyl-
CoA to thiogalactosidase.
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6. Induction
lacI P O lacZ lacY lacA
Repressor
Inducer
Available
Transcription
RNA
Pol
Confirmation changed(Allo-
lactose)
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7. Repression
lacI P O lacZ lacY lacA
Repressor
No Inducer
Available
NoTranscription
RN
A
Pol
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8. Glucose, cAMP and CAP
 Glucose primary source of energy for all
organisms.
 cAMP – Cyclic Adenosine monophosphate.
 CAP – Catabolite activation protein.
 CBS – CAP binding site.
 CBS present in the upstream of the lac promoter.
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9. Contd…
Case 1: Glucose present; Lactose absent
 Low cAMP, thus CAP does not binds to DNA as well as RNA
polymerase.
 Repressor remains in the operator region and inhibits the
transcription.
Case 2: Glucose present; lactose present
 Lac repressor is released from operator region.
 But cAMP levels are low, thus CAP is inactive.
 So, low level of transcription.
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10. lacI P O lacZ lacY lacA
CBS
High glucose
ATP cAMP
EIIA
Adenylyl
cyclase
activates cAMP
CAP
NO transcription
NO Lactose
Case 1: Glucose present; Lactose absent
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11. Case 2: Glucose present; lactose present
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12. Contd…
Case 3: Glucose absent; lactose absent
 High cAMP levels; CAP is active.
 However, repressor still binds to operator region and no
transcription takes place.
Case 4: Glucose absent; lactose present
 Repressor is released from the operator region.
 cAMP is high; activates CAP; RNA polymerase binds to DNA,
strong transcription occurs.
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13. lacI P O lacZ lacY lacA
CBS
NO glucose
ATP cAMP
EIIA
Adenylyl
cyclase
activates cAMP
CAP
NO Transcription
NO
Lactose
Case 3: Glucose absent; lactose absent
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14. Case 4: Glucose absent; lactose present
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15. Glucose Lactose CAP
binding
Repressor
binding
Result
Present Absent No binding Binding
occurs
No
transcription
Present Present No binding No binding Low level
transcription
Absent Absent Binding
occurs
Binding
occurs
No
transcription
Absent Present Binding
occurs
No binding Strong
transcription
SUMMARY
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16. THANK YOU
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