Transcript 5 Bacterial Gene Control 2011

Control of
Prokaryotic (Bacterial) Genes
(Watch this Video!)
2007-2008
Bacterial metabolism and regulation

Bacteria need to respond quickly to
changes in their environment

For example, if they have enough of a
product, they need to stop production

STOP

GO

WHY? waste of energy to produce more
HOW? They stop production of enzymes need
for synthesis of the product
Also…if they find new food/energy source,
they need to utilize it quickly


WHY? For metabolism, growth, reproduction
HOW? They start producing the enzymes for
digestion
How is metabolism (in general)
regulated?
Recall: Feedback inhibition
The PRODUCT acts
as an allosteric inhibitor of
the 1st enzyme in
(tryptophan) pathway
-
= inhibition
-
Bacteria are different in their
approach to regulating metabolism
They use: Gene regulation
-
Instead of just blocking
enzyme function, they
block transcription of genes
for all enzymes in pathway
= inhibition
This saves energy by
not wasting it on
unnecessary protein
synthesis
-
-
Cells can then vary the amount of specific
enzymes by regulating gene transcription

They turn genes on or turn genes off
turn genes OFF example
STOP if bacterium has enough tryptophan then it doesn’t
need to make enzymes used to build tryptophan
 turn genes ON example
if bacterium encounters new sugar (energy
GO source), like lactose, then it needs to start making
enzymes used to digest lactose

Bacteria group genes together in
clusters called operons.
These are genes grouped together with related
functions
For example: all enzymes in a metabolic pathway
There are TWO means of control in an operon:

promoter = RNA polymerase binding site


single promoter controls transcription of all genes in
operon
transcribed as one unit & a single mRNA is made
operator = DNA binding site of repressor protein
How are genes turned off?
Repressor proteins…
binds to DNA at operator site
 blocking RNA polymerase
 blocks transcription

Overview of Operon model
What is an ‘Operon’ ?
Includes: operator, promoter & genes they control
Serves as a model for gene regulation
RNA
polymerase
RNA
repressor
TATA
polymerase
gene1
gene2
gene3
gene4
1
2
3
4
enzyme1
enzyme2
enzyme3
enzyme4
mRNA
promoter
DNA
operator
Repressor protein turns off gene by
blocking RNA polymerase binding site.
repressor
= repressor protein
Example of repressible operon:
tryptophan
Synthesis pathway model
When excess tryptophan is present,
it binds to tryp repressor protein &
triggers repressor to bind to DNA
RNA
polymerase

RNA
trp repressor
TATA
polymerase
gene1
gene2
gene3
gene4
1
2
3
4
enzyme1
enzyme2
enzyme3
enzyme4
mRNA
promoter
blocks (represses) transcription
DNA
trp
operator
trp
trp
repressor
repressor protein
trp
trp
trp
trp
trp
trp
conformational change in
repressor protein!
trp
repressor
tryptophan
trp
tryptophan – repressor protein
complex
Conclusion: Tryptophan is allosteric regulator of repressor protein
What about the opposite? What about
‘Inducible’ operons? Example: lactose
lac
lac
RNA
polymerase
lac
Digestive pathway model
lac
When lactose is present, binds to
lac repressor protein & triggers
repressor to release DNA
lac
lac
lac
RNA
lac repressor
TATA
polymerase
induces transcription
gene1
gene2
gene3
gene4
1
2
3
4
enzyme1
enzyme2
enzyme3
enzyme4
mRNA
promoter

operator
repressor
Caused by a
conformational change in
repressor protein!
lac
lac
repressor
DNA
repressor protein
lactose
lactose – repressor protein
complex
Lactose operon
What happens when lactose is present?
Need to make lactose-digesting enzymes
Lactose is allosteric regulator of repressor protein
1961 | 1965
Jacob & Monod: lac Operon

Francois Jacob & Jacques Monod
first to describe operon system
 coined the phrase “operon”

Jacques Monod
Francois Jacob
Operon summary

Repressible operon

usually functions in anabolic pathways



synthesizing end products
when end product is present in excess,
cell allocates resources to other uses
Inducible operon

usually functions in catabolic pathways,


digesting nutrients to simpler molecules
produce enzymes only when nutrient is
available

cell avoids making proteins that have nothing to do,
cell allocates resources to other uses
Don’t be repressed!
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2007-2008