The lac operon – An inducible operon
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Transcript of The lac operon – An inducible operon
The lac operon – An inducible operon
• Genes are either cis (coupling) acting– Genes affect those adjacent to themselves– Operator and structural genes
Or
• trans (repulsion) acting– Genes affect other genes that are not adjacent to
themselves– Implies the action of a diffusible product– Regulator genes
Trans (repulsion) acting genes•Genes that affect other genes•Are not adjacent to the genes they control–Regulator genes
Implies the action of a diffusible product
Genes that are cis (coupling) acting
•Genes affect genes adjacent to themselves
Operator and structural genes
The lac operon – An inducible operon
• Bacteria are monoploid – one copy of each gene
• Can make bacteria partially diploid (two copies of a gene)– Place another copy of the gene(s) on a plasmid
(F’)
– E.coli lac I mutant with a plasmid carrying a functional repressor gene will be designated: I - O + Z +/F’ I +
The lac operon – An inducible operon
beta-galtosidase activity
beta-galactosidase
activity
Genotype Lactose present Lactose absent
I + O + Z + + -
I - O + Z +/F’ I +
I + O c Z +/F’ I +
I + O + Z +/ F’ I s
I s O c Z +/F’ I +
I s O c Z -/F’ I -
The lac operon – An inducible operon
beta-galtosidase activity
beta-galactosidase
activity
Genotype Lactose present Lactose absent
I + O + Z + + -
I - O + Z +/F’ I +
I + O c Z +/F’ I +
I + O + Z +/ F’ I s
I s O c Z +/F’ I +
I s O c Z -/F’ I -
The lac operon – An inducible operon
beta-galtosidase activity
beta-galactosidase
activity
Genotype Lactose present Lactose absent
I + O + Z + + -
I - O + Z +/F’ I +
I + O c Z +/F’ I +
I + O + Z +/ F’ I s
I s O c Z +/F’ I +
I s O c Z -/F’ I -
The lac operon – An inducible operon
beta-galtosidase activity
beta-galactosidase
activity
Genotype Lactose present Lactose absent
I + O + Z + + -
I - O + Z +/F’ I + + -
I + O c Z +/F’ I +
I + O + Z +/ F’ I s
I s O c Z +/F’ I +
I s O c Z -/F’ I -
The lac operon – An inducible operon
beta-galtosidase activity
beta-galactosidase
activity
Genotype Lactose present Lactose absent
I + O + Z + + -
I - O + Z +/F’ I + + -
I + O c Z +/F’ I + + +
I + O + Z +/ F’ I s
I s O c Z +/F’ I +
I s O c Z -/F’ I -
plasmid
E. Coli genome
The lac operon – An inducible operon
beta-galtosidase activity
beta-galactosidase
activity
Genotype Lactose present Lactose absent
I + O + Z + + -
I - O + Z +/F’ I + + -
I + O c Z +/F’ I + + +
I + O + Z +/ F’ I s - -
I s O c Z +/F’ I +
I s O c Z -/F’ I -
The lac operon – An inducible operon
beta-galtosidase activity
beta-galactosidase
activity
Genotype Lactose present Lactose absent
I + O + Z + + -
I - O + Z +/F’ I + + -
I + O c Z +/F’ I + + +
I + O + Z +/ F’ I s - -
I s O c Z +/F’ I + + +
I s O c Z -/F’ I -
The lac operon – An inducible operon
beta-galtosidase activity
beta-galactosidase
activity
Genotype Lactose present Lactose absent
I + O + Z + + -
I - O + Z +/F’ I + + -
I + O c Z +/F’ I + + +
I + O + Z +/ F’ I s - -
I s O c Z +/F’ I + + +
I s O c Z -/F’ I - - -
The lac operon – An inducible operon
In F’ plasmid, an entire lac operon can be supplied. For example,
I + O + Z - Y +/F’ I - O + Z + Y -
What are the beta-galactosidase and permease activities with and without lactose in the E. coli strain with the above genotype?
The lac operon
• If E. coli cells are provided with both glucose and lactose, it preferentially metabolizes glucose rather than lactose. How do E. coli cells accomplish this?
• Positive control and catabolite repression
Positive Control and Catabolite Repression
New Players:Cyclic AMP (cAMP)
CAP (catabolite activator protein)
High glucose low cAMP
Low glucose high cAMP
Positive Control and Catabolite Repression
High glucose low cAMP
Low glucose high cAMP
Positive Control and Catabolite Repression
High glucose low cAMP
Low glucose high cAMP
Positive Control and Catabolite Repression
The cAMP-CAP complex interacts with the promoter
(neither can bind the promoter of lac operon by themselves)
CAP = Catabolite Activator Protein
Facilitates the attachment of the RNA polymerase to the promoter – acts as an activator
Must be a cAMP-CAP complex to bind to the promoter
Positive Control and Catabolite Repression
What would happen to the lac operon when:
1. Glucose present (+), but lactose absent (-)
(low)
No transcription
The cAMP-CAP complex is not formed (low cAMP), so CAP does not bind to the promoter
In addition, there is no inducer (lactose), so the active repressor is bound to the operator, and therefore the RNA polymerase cannot bind and transcribe the Lac operon genes
Positive Control and Catabolite Repression
What would happen to the lac operon when:
1. Glucose present (+), but lactose absent (-)– No transcription
Positive Control and Catabolite Repression
What would happen to the lac operon when:
1. Glucose present (+), but lactose absent (-)– No transcription
2. Glucose and lactose both present (+)
The cAMP-CAP complex does not form (low cAMP)
The inducer (lactose) is present and inactivates the repressor•There is some transcription, but it is inefficient (no cAMP-CAP complex)
Positive Control and Catabolite Repression
What would happen to the lac operon when:
1. Glucose present (+), but lactose absent (-)– No transcription
2. Glucose and lactose both present (+)– Very little transcription
Positive Control and Catabolite Repression
What would happen to the lac operon when:
1. Glucose present (+), but lactose absent (-)– No transcription
2. Glucose and lactose both present (+)– Very little transcription
3. Glucose absent (-), but lactose present (+)
The cAMP-CAP complex does form (high cAMP), facilitating the attachment of the RNA polymerase to the promoter
The RNA polymerase is effectively attached to the promoter, enhancing transcription
Positive Control and Catabolite Repression
What would happen to the lac operon when:
1. Glucose present (+), but lactose absent (-)– No transcription
2. Glucose and lactose both present (+)– Very little transcription
3. Glucose absent (-), but lactose present (+) – A lot of transcription
Positive Control and Catabolite Repression
What would happen to the lac operon when:
1. Glucose present (+), but lactose absent (-)– No transcription
2. Glucose and lactose both present (+)– Very little transcription
3. Glucose absent (-), but lactose present (+)– A lot of transcription
4. Glucose and lactose both absent (-)
No inducer (no lactose) No transcriptionRepressor stays bound to the operator
cAMP pairs with CAP and can attach to the promoter
Positive Control and Catabolite Repression
What would happen to the lac operon when:1. Glucose present (+), but lactose absent (-)
– No transcription2. Glucose and lactose both present (+)
– Very little transcription3. Glucose absent (-), but lactose present (+)
– A lot of transcription4. Glucose and lactose both absent (-)
– No transcription
Lac operon overview - animation
http://highered.mcgraw-hill.com/sites/0072556781/student_view0/chapter12/animation_quiz_4.html