Transcript Bacterial Gene Regulation: Trp & Lac Operon Model

Ch 18
Gene Regulation
Consider:
A multicellular organism (Pliny)
Do each of his cells have the same genes?
Yes, with an exception:
germ cells are haploid
Do each of his cells express the same genes?
Examples to support your claim
Do cells express the same genes at the same time and at the
same level? Examples
How are Genes Regulated?
LE 18-20
How might genes be regulated?
Regulation of enzyme
activity
Precursor
Regulation of enzyme
production
Feedback
inhibition
Enzyme 1
Gene 1
Enzyme 2
Gene 2
Regulation
of gene
expression
Enzyme 3
Gene 3
Enzyme 4
Gene 4
Enzyme 5
Gene 5
Tryptophan
Give an example of how proteins are regulated.
Bacterial Operons:
Gene Regulation Model
Genes grouped into operons
- Promoter to help initiate transcription
- Operator: DNA sequence acts as on-off switch
- Genes encode metabolic enzymes
Operon regulated by repressors and/or activators
in response to environment.
Trp Operon
Group of genes that encode enzymes
for tryptophan synthesis (an amino acid)
RNA polymerase
DNA
mRNA
proteins
trpE
trpD
trpC
trpB
trpA
E
D
C
B
A
5’
LE 18-21a
Trp Operon ON most of the time
TrpR gene also ON: makes inactive repressor protein
trp operon
Promoter
Promoter
Genes of operon
DNA
Regulatory
gene
mRNA
trpE
trpR
3
trpC
trpB
trpA
C
B
A
Operator
Start codon Stop codon
RNA
polymerase
mRNA 5
5
E
Protein
trpD
Inactive
repressor
D
Polypeptides that make up
enzymes for tryptophan synthesis
Tryptophan absent, repressor inactive, operon on
If the cell is not synthesizing much protein (e.g. low nutrients),
will it need to continue to make trp?
How to shut off the trp operon?
LE 18-21b_1
DNA
mRNA
Active
repressor
Protein
Tryptophan
(corepressor)
Tryptophan present, repressor active, operon off
LE 18-21b_2
operator
TrpR
DNA
No RNA made
mRNA
conformational
Protein
change
Tryptophan
(corepressor)
Tryptophan present, repressor active, operon off
Active
repressor
Is the trp operon repressible or inducible?
ON unless excess trp binds and activates repressor protein->
Active TrpR binds operator
Blocks transcription
Trp operon OFF
Do inducible operons exist?
(usually OFF; need signal to turn ON
Lac operon:
group of genes involved in catabolism of lactose
LE 18-22b
Lac operon
DNA
lacZ
lacl
3
mRNA
5
lacA
Permease
Transacetylase
RNA
polymerase
mRNA 5
-Galactosidase
Protein
Allolactose
(inducer)
lacY
Inactive
repressor
Lactose present, repressor inactive, operon on
Enzymes facilitate lactose import and breakdown
for cellular energy
Let’s assume bacteria prefer glucose to lactose as a
carbon source.
If glucose is available in the surroundings, does it make
sense for the lac operon to be ON?
If it’s a waste of energy then how do bacteria repress (turn OFF)
the Lac operon?
LE 18-22a
Promoter
Regulatory
gene
Operator
lacl
DNA
lacZ
No
RNA
made
3
mRNA
5
Protein
RNA
polymerase
Active
repressor
Lactose absent, repressor active, operon off
• Inducible gene products
– usually function in catabolic pathways (lactose
metabolism)
• Repressible gene products
-products usually function in anabolic pathways
(trp synthesis)
Trp and lac operons (similarities):
- Negatively controlled
- Blocked by a repressor
Positive Gene Regulation
• Activator protein turns on Lac operon
– catabolite activator protein (CAP)
Glucose high cAMP low
Glucose low cAMP high
CAP-cAMP binds Lac promoter and induces transcription
When would this occur, when glucose is high or low?
Low
LE 18-23a
Promoter
DNA
lacl
lacZ
CAP-binding site
Active
CAP
cAMP
Inactive
CAP
RNA
Operator
polymerase
can bind
and transcribe
Inactive lac
repressor
Lactose present, glucose scarce (cAMP level high): abundant lac
mRNA synthesized
LE 18-23b
Promoter
DNA
lacl
CAP-binding site
Inactive
CAP
lacZ
Operator
RNA
polymerase
can’t bind
Inactive lac
repressor
Lactose present, glucose present (cAMP level low): little lac
mRNA synthesized
Oh gee, am I supposed to induce or repress?
Do you have questions too?
LE 18-21b_1
DNA
mRNA
Active
repressor
Protein
Tryptophan
(corepressor)
Tryptophan present, repressor active, operon off
LE 18-23a
Promoter
DNA
lacl
lacZ
CAP-binding site
Active
CAP
cAMP
Inactive
CAP
RNA
Operator
polymerase
can bind
and transcribe
Inactive lac
repressor
Lactose present, glucose scarce (cAMP level high): abundant lac
mRNA synthesized
A mutation arises in the TrpR gene that inactivates the binding
site for the co-repressor. How will the mutant phenotype differ
from wildtype?
If the operator is deleted from the lac operon predict how gene
expression will be altered relative to wildtype under the following
conditions
a. lactose present, glucose absent
b. lactose absent, glucose present
c. lactose present, glucose present
d. lactose absent, glucose absent