Transcript Transcription/Translation

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
Genotype
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 -
Lactose present
+
betagalactosidase
activity
Lactose absent
-
The lac operon – An inducible operon
beta-galtosidase
activity
Genotype
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 -
Lactose present
+
betagalactosidase
activity
Lactose absent
-
The lac operon – An inducible operon
beta-galtosidase
activity
Genotype
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 -
Lactose present
+
betagalactosidase
activity
Lactose absent
-
The lac operon – An inducible operon
beta-galtosidase
activity
Genotype
Lactose present
betagalactosidase
activity
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
Genotype
Lactose present
betagalactosidase
activity
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
Genotype
Lactose present
betagalactosidase
activity
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
Genotype
Lactose present
betagalactosidase
activity
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
Genotype
Lactose present
betagalactosidase
activity
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)
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
High glucose
low cAMP
Low glucose
high cAMP
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 (-)
The cAMP-CAP complex is not
formed (low cAMP), so CAP
does not bind to the promoter
(low)
No transcription
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 (-)
cAMP pairs with CAP and can attach to the promoter
No inducer (no lactose)
Repressor stays bound to the operator
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 (+)
– Very little transcription
3. Glucose absent (-), but lactose present (+)
– A lot of transcription
4. Glucose and lactose both absent (-)
– No transcription
Lac operon overview - animation
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z_4.html