Transcript Control of Gene Expression

Control of Gene
Expression
More about bacteria!
• Now part of the rest of the bacteria chromosome
can be transferred to another bacteria cell:
•
Recombination exchanges segments of DNA.
Why would a bacteria want to
control gene expression anyway?

In order to adjust to the changing
environment.
• For example:
•

Amino acid, Tryptophan
(found in chicken, turkey, fish, soybeans,
etc.)
E. coli can make their own tryptophan, but are
they going to bother making their own when
Thanksgiving rolls around?!
But first…some VOCABULARY!
Operon


The entire stretch of DNA required for
enzyme production.
Includes the operator, promoter and
genes they control.
Promoter

Region where RNA polyermase binds
Operator


Acts as an “on-off switch”
Region between the promoter and the
first gene
Repressor

Protein that binds to the operator and
blocks the attachment of RNA
polymerase
Corepressor

A small molecule that cooperates with a
repressor protein to switch an operator
off
Regulatory Gene

The gene that produces repressors
Inducer

A molecule that inactivates the repressor
Now let’s put it all together!
How do all the parts interact?
NO tryptophan in environment:
•
•
•
Repressor does not bind to the
operator
RNA polymerase attaches
Enzymes are made that make
tryptophan
What happens if we just ate
some turkey?!
Tryptophan in the environment:
• Tryptophan acts as a corepressor and
binds to the repressor.
• The repressor now has the correct
shape to bind to the operator.
• RNA polymerase can NOT attach to the
promotor
• Tryptophan synthesis is STOPPED!
Repressible vs. Inducible
Operons

Repressible operon: one that is inhibited
when a specific small molecule binds to a
regulatory protein
• Normally ON: active
• Function in anabolic pathways (making)

Inducible operon: one that is stimulated
when a specific small molecule interacts with
a regulatory protein.
• Normally OFF: inactive
• Function in catabolic pathways (breaking down)
Inducible operon
example…

The lac operon contains genes that
code for enzymes that break down
lactose.
• Your mom forgot to buy milk: your
bacteria are not going to make any
enzymes for the breakdown of
lactose (OFF)
No milk = operator OFF
Got milk = operator ON
Do you know your
vocabulary?!
Let’s find out!
Clue #1


The entire stretch of DNA required for
enzyme production.
Includes the operator, promoter and
genes they control.
Operon
Clue #2

Region where RNA polyermase binds
Promoter
Clue #3


Acts as an “on-off switch”
Region between the promoter and the
first gene
Operator
Clue #4

Protein that binds to the operator and
blocks the attachment of RNA
polymerase
Repressor
Clue #5

A small molecule that cooperates with a
repressor protein to switch an operator
off
Corepressor
Clue #6

The gene that produces repressors
Regulatory Gene
Clue #7

A molecule that inactivates the repressor
Inducer