Ch 18

1
Ch 18
Gene Regulation
2
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
3
How are Genes Regulated?
4
LE 18-20
How might genes be regulated?
Regulation of enzyme production
Regulation of enzyme activity
Precursor
Feedback inhibition
Enzyme 1
Gene 1
Enzyme 2
Gene 2
Regulation of gene expression
Gene 3
Enzyme 3
Enzyme 4
Gene 4
Gene 5
Enzyme 5
Tryptophan
Give an example of how proteins are regulated.
5
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.
6
Trp Operon
Group of genes that encode enzymes for
tryptophan synthesis (an amino acid)
RNA polymerase
7
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
trpE
trpC
trpB
trpA
trpR
trpD
Operator
Stop codon
RNA polymerase
Regulatory gene
Start codon
3
mRNA 5
mRNA
5
D
B
E
C
A
Protein
Inactive repressor
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?
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LE 18-21b_1
DNA
mRNA
Protein
Active repressor
Tryptophan (corepressor)
Tryptophan present, repressor active, operon off
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LE 18-21b_2
operator
TrpR
DNA
No RNA made
mRNA
conformational
Protein
Active repressor
change
Tryptophan (corepressor)
Tryptophan present, repressor active, operon off
10
Is the trp operon repressible or inducible?
ON unless excess trp binds and activates
repressor protein-gt
Active TrpR binds operator Blocks
transcription Trp operon OFF
11
Do inducible operons exist? (usually OFF need
signal to turn ON
Lac operon group of genes involved in
catabolism of lactose
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LE 18-22b
Lac operon
DNA
lacl
lacZ
lacY
lacA
RNA polymerase
3
mRNA
mRNA 5
5
Permease
Transacetylase
?-Galactosidase
Protein
Inactive repressor
Allolactose (inducer)
Lactose present, repressor inactive, operon on
Enzymes facilitate lactose import and
breakdown for cellular energy
13
Lets 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 its a waste of energy then how do bacteria
repress (turn OFF) the Lac operon?
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LE 18-22a
Promoter
Regulatory gene
Operator
lacl
lacZ
DNA
No RNA made
3
mRNA
RNA polymerase
5
Active repressor
Protein
Lactose absent, repressor active, operon off
15

• 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
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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
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LE 18-23a
Promoter
DNA
lacl
lacZ
RNA polymerase can bind and transcribe
Operator
CAP-binding site
Active CAP
cAMP
Inactive lac repressor
Inactive CAP
Lactose present, glucose scarce (cAMP level
high) abundant lac mRNA synthesized
18
LE 18-23b
Promoter
DNA
lacl
lacZ
CAP-binding site
Operator
RNA polymerase cant bind
Inactive CAP
Inactive lac repressor
Lactose present, glucose present (cAMP level
low) little lac mRNA synthesized
19
Oh gee, am I supposed to induce or repress?
Do you have questions too?
20
LE 18-21b_1
DNA
mRNA
Protein
Active repressor
Tryptophan (corepressor)
Tryptophan present, repressor active, operon off
21
LE 18-23a
Promoter
DNA
lacl
lacZ
RNA polymerase can bind and transcribe
Operator
CAP-binding site
Active CAP
cAMP
Inactive lac repressor
Inactive CAP
Lactose present, glucose scarce (cAMP level
high) abundant lac mRNA synthesized
22
A mutation arises in the TrpR gene that
inactivates the bindingsite 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