Transcript Control of Gene Activity

Control of Gene
Activity
Chapter 18: Regulation of Gene Expression
Gene Regulation
 Prokaryotes and eukaryotes
alter gene expression in
response to their
 In multicellular eukaryotes,
gene expression regulates
development and is
responsible for differences in
cell types

molecules play many
roles in regulating gene
expression in eukaryotes
Prokaryotic regulation
 Gene expression in bacteria is
controlled by the operon model
 An
is a cluster of
functionally related genes can be
under coordinated control by a
single on-off “switch”
Bacteria do not require same
enzymes all the time
They produce just enzymes
needed at the moment
 The regulatory “switch” is a
segment of DNA called an
(positioned
within the promoter)
Prokaryotic regulation
 The operon can be switched off by a
protein
 The repressor prevents gene
transcription by binding to the
operator and blocking RNA
polymerase
 The repressor is the product of a
separate
 The repressor can be in an active or
inactive form, depending on the
presence of other molecules
A
is a molecule
that cooperates with a repressor
protein to switch an operon off
Operon Components
 The operon includes the following:
1)
Located outside the operon
Codes for a repressor protein molecule
2)
Sequence of DNA where RNA polymerase attaches
 3)
A short sequence of DNA where repressor binds,
preventing RNA polymerase from binding
4)
Code for enzymes of a metabolic pathway
Transcribed as a unit
E. coli & Tryptophan
 E. coli is a bacteria that lives in your
colon
 It has a metabolic pathway that
allows for the synthesis of the amino
acid tryptophan (Trp)
 This pathway starts with a precursor
molecule and proceeds through
before
reaching the final product:
tryptophan
 It is important that E. coli be able to
control the rate of Trp synthesis
because the amount of Trp available
from the environment varies
considerably
E. coli & Tryptophan
 If you eat a meal with little or no
Trp, the E. coli in your gut must
compensate by making more
 If you eat a meal rich in Trp, E. coli
doesn't want to waste valuable
resources or energy to produce the
amino acid because it is readily
available for use
 Therefore, E. coli uses the amount
of Trp present to regulate the
pathway
 If levels are not adequate, the rate of
Trp synthesis is
 If levels are adequate, the rate of
Trp synthesis is
trp Operon
 The Trp operon has three components:

 These genes contain the genetic code for the five
enzymes in the Trp synthesis pathway

 DNA segment where RNA polymerase binds and
starts transcription

 DNA segment found between the promoter and
structural genes
 It determines if transcription will take place
trp Operon
trp operon
 When nothing is bonded to the operator, the operon is
RNA polymerase binds to the promoter and transcription is
initiated
The 5 structural genes are transcribed to one mRNA strand
The mRNA will then be translated into the 5 enzymes that
control the Trp synthesis pathway
trp operon
The operon is turned
by a specific
protein called the
The repressor is inactive in this form and can not
bind properly to the operator
To become active and bind properly, a corepressor
must associate with the repressor
The corepressor for this operon is Tryptophan
This makes sense because E. coli does not want to
synthesize Trp if it is available from the
environment
trp operon
trp operon
 An active repressor binds to the operator blocking the
attachment of
to the promoter
 Without RNA polymerase, transcription and translation
of the structural genes can't occur and the enzymes
needed for Tryptophan synthesis are not made
 By default the trp operon is
and the genes for
tryptophan synthesis are
 When tryptophan is present, it binds to the trp repressor
protein, which turns the operon off
 The repressor is active only in the presence of its
corepressor tryptophan; thus the trp operon is repressed
if tryptophan levels are high
Repressible vs Inducible
 The trp operon is a
This type is one that is usually on
Binding of a repressor to the operator shuts off
transcription
The end product, Trp, decreases or stops the transcription
of the enzymes necessary for its production
 The opposite is called an
This type is one that is usually off
A molecule called an
inactivates the repressor and
turns on transcription
An example of an inducible system is lac (lactose) operon
lac operon
 The Lac operon has 3 components:

Contain the genetic code for the 3 enzymes in the
lactose catabolic pathway

DNA segment where RNA polymerase binds and starts
transcription

DNA segment found between the promoter and
structural genes
It determines if transcription will take place
If the operator in turned "on", transcription will occur
lac operon
 The lac operon is an inducible
operon whose genes code for
enzymes used in the hydrolysis
and metabolism of lactose
 By itself, the lac repressor is
active and switches the lac
operon off
 The active repressor binds
to the operator
 This blocks RNA
polymerase from
transcribing the genes

lac operon OFF
lac operon
 A molecule called an
inactivates
the repressor to turn the lac operon on
 What inducer is used in the lac operon?
This makes sense because the cell only needs to
make enzymes to catabolize lactose if lactose is
present
When lactose enters the cell it binds to the repressor
and changes its shape so that it can't bind to the
operator
 RNA polymerase can now start transcription of the 3
structural genes that will control lactose catabolism
lac operon ON
Eukaryotic Regulation
 Eukaryotes lack a
regulatory mechanism to
control expression of
genes coding proteins
 In multicellular organisms
gene expression is
essential for
 DNA in eukaryotes is
packaged as chromatin
within a nucleus
Chromatin in Eukaryotes
 Most chromatin is loosely
packed in the nucleus during
interphase and condenses prior
to mitosis
 Loosely packed chromatin is
called
 The genes within this area
are easily accessed, thus
easily transcribed
 During interphase a few
regions of chromatin are highly
condensed into
 Genes within this area are
difficult to access, thus they
are usually not transcribed
Eukaryotic regulation
There are four primary levels of control of
gene activity:
1. Transcriptional Control
2. Posttranscriptional Control
3. Translational Control
4. Posttranslational Control
Transcriptional Control
 Takes place in
,
the site of transcription
 Determines which structural
genes are transcribed and
the rate of transcription
 Includes organization of
 Includes the action of
may activate or inhibit
transcription (such as
transcription factors)
that
Regulatory Proteins
 General transcription factors are essential for the
transcription of all protein-coding genes
 Some transcription factors function as
An activator is a protein that binds to an enhancer and
stimulates transcription of a gene

are DNA sequences that may be far
away from a gene or even located in an intron
 Some transcription factors function as
A repressor is a protein that prevents the expression of a
particular gene
 Some activators and repressors act indirectly by influencing
chromatin structure to promote or silence transcription
Posttranscriptional control
 Takes place in the
 Involves
Differential excision of introns and splicing of mRNA can
vary type of mRNA
In alternative RNA splicing, different mRNA molecules
are produced from the same primary transcript, depending
on which RNA segments are treated as exons and which as
introns
 Involves regulation of
The life span of mRNA molecules in the cytoplasm is a key
to determining protein synthesis
The mRNA life span is determined in part by sequences in
the leader and trailer regions (cap and tail)
Translational control
 Takes place in the
,
the site of translation
 Life expectancy of mRNA
molecules can vary, as well as
their ability to bind ribosomes
 Some mRNA's may need
additional changes before they
are translated
 The initiation of translation of
selected mRNAs can be
blocked by regulatory proteins
that bind to sequences or
structures of the mRNA
Posttranslational control
Takes place in the
after
May involve
of the
protein
After translation, various types of protein
processing, including cleavage and the addition
of functional groups, are subject to control

are giant protein complexes
that bind protein molecules and degrade them
Eukaryotic Gene Control Animation
Review Questions
1.
2.
3.
4.
Differentiate between prokaryotic and eukaryotic gene regulation.
Explain the use of an operon as a prokaryotic form of gene regulation.
Name and describe the four main parts of an operon.
Define the following terms: operator, repressor, inducer, regulatory
gene, and corepressor.
5. Describe the functioning of the trp operon as a repressible operon
and state its overall significance to E. coli.
6. Differentiate between repressible and inducible operons.
7. Describe the functioning of the lac operon as an inducible operon and
state its overall significance to E. coli.
8. Differentiate between euchromatin and heterochromatin in
eukaryotes.
9. Name and describe the important traits of the 4 primary levels of
control of gene activity in eukaryotes.
10. Differentiate between activators, enhancers, and repressors.
11. Describe alternative RNA splicing and its significance to gene control.
12. Define proteasome.