Transcript operon

Regulation of Gene Expression – Part I
Spring 2013 - Althoff Reference: Mader & Windelspecht Ch. 13)
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Lec
18
Prokaryotic
Regulation
Prokaryotic Regulation
• Bacteria do not always ________ their entire
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complement of enzymes and proteins
…because their environment is ever changing.
In 1961, French microbiologists Francois Jacob
and Jacques Monod showed that Escherichia
coli is capable of regulating the expression of
its genes.
They observed that ________________ for a
__________________ are grouped on a
chromosome…and transcribed at the same
time
The ___________ Model
• Jacob and Monod proposed a model to explain
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gene regulation in prokayotes
An operon has these components:
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4
1
5
2
The Operon Elements
1• _______________ – normally located outside
the operon. Codes for the DNA-binding protein
that acts as a ______________ 2
3• _____________ - a _______________ of DNA
where RNA polymerase first attaches to begin
transcription (remember this happens in the
nucleus) of the grouped genes.
The Operon Elements
4• __________– a short portion of DNA where an
active repressor binds. When present, RNA
polymerase _____________ to the promoter—
thus ____ transcription possible
5
• _________________– genes that ______ for
the enzymes and proteins that are involved in
the metabolic pathway of the operon
The trp Operon: on vs. off
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Jacob and Monod…and others…found that some E.
coli operons usually existed in the “___” condition vs.
the “____” condition
This means tryptophan is not presence and the
active repressor is “inactive”
When “___”, their products—5 different enzymes—
are part of _________________ for the synthesis of
the amino acid tryptophan.
If tryptophan present in the medium, then these
enzymes are not needed…and the tryptophan _____
to the repressor making it an __________________
The trp operon – making A or not B making tryptophan
A
Tryptophan ABSENT…enzymes needed are produced
▲Tryptophan
ABSENT…needed enzymes not produced
B
The trp Operon: on vs. off…con’t
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When the trytophan binds to the repressor, it
________________…result: RNA polymerase can’t
bind to the polymerase
The lac Operon
• Bacteria metabolism is very efficient…when there is
___________ for certain proteins or enzymes, the
genes needed to make them are usually inactive.
• Example: if the milk sugar lactose is not present,
there is no need to “___________” genes for
enzymes involve in lactose ____________.
• There are ________ encoded for the enzymes
needed to break down lactose: β-galatosidase,
permease, and transacetylase.
The lac Operon…con’t
• The 3 structural genes that code for producing those
enzymes are ____________ to one another…
• …and are under control of a __________ promoter
and a ___________ operator.
• Because the presence of lactose brings about
expression of genese, it is called an ___________ of
the lac operon
• The entire “unit” is called an ________________
because they have “inducible” enzymes.
Further Control of the lac Operon
• E. coli preferentially breaks
down glucose…and the
bacterium has a way to ensure
that the lactose operon is
maximally turned on ______
when glucose is absent.
• Cyclic AMP (cAMP)
_____________ when glucose
is absent.
• cAMP, which is a derivative of
ATP, has a single phosphate
group—which is attached to the
ribose at two locations
More on cAMP…
• cAMP binds to a molecule called catabolite activator
protein = _____…and that complex attaches to the
CAP binding site next to the lac promoter.
• When CAP binds to DNA, ____________, exposing
the promoter to RNA polymerase….
• …the result: RNA polymerase is now __________ to
bind to the promoter aiding transcription which
leads to their expression
Lactose present, glucose absent…cAMP level _______
Lactose present, glucose absent…cAMP level _____
Positive Control
• CAP’s protein regulation of the lac operon is an
example of _________ control.
• Why? When CAP is active it ___________ the
activity of the operon.
Negative Control
• The use of repressors is an example of a _________
control
• Why? When the repressor is active ____________
the operon.
Positive Control
of gene expression
Negative Control
of gene expression