Gene Regulation
Chapter 14
Learning Objective 1
•
Why do bacterial and eukaryotic cells have
different mechanisms of gene regulation?
Prokaryotes
•
Bacterial cells
•
•
•
grow rapidly
have a short life span
Transcriptional-level control
•
usually regulates gene expression
Eukaryotic Cells
•
Have long life span
•
•
One gene
•
•
respond to many different stimuli
may be regulated in different ways
Transcriptional-level control
•
and control at other levels of gene expression
KEY CONCEPTS
•
Cells can synthesize thousands of proteins
•
•
but not all proteins are required in all cells
Cells regulate which parts of the genome
will be expressed, and when
Learning Objective 2
•
What is an operon?
•
What are the functions of the operator and
promoter regions?
Operon
•
A gene complex
•
•
•
structural genes with related functions
controlled by closely linked DNA sequences
Regulated genes in bacteria
•
are organized into operons
Promoter Region
•
Each operon has a promoter region
•
•
upstream from protein-coding regions
where RNA polymerase binds to DNA before
transcription
Operator (1)
•
Regulatory switch for transcriptional-level
control of operon
•
Repressor protein
•
•
binds to operator sequence
prevents transcription
Operator (2)
•
RNA polymerase
•
•
•
bound to promoter
is blocked from transcribing structural genes
If repressor is not bound to operator
•
transcription proceeds
Learning Objective 3
•
What is the difference between inducible,
repressible, and constitutive genes?
Inducible Genes (1)
•
An inducible operon
•
•
•
such as lac operon
is normally turned off
Repressor protein
•
•
is synthesized in active form
binds to operator
Inducible Genes (2)
•
If lactose is present
•
•
•
•
is converted to allolactose (inducer)
binds to repressor protein
changes repressor’s shape
Altered repressor
•
•
cannot bind to operator
operon is transcribed
The lac Operon
lac operon
Repressor
gene
Promoter Operator
lac Z
lac Y lac A
DNA
Repressor
protein
Transcription
mRNA
Ribosome
Translation
Fig. 14-2a, p. 307
Repressor
gene
mRNA
lac operon
Promoter Operator
lac Z lac Y
RNA
polymerase
lac A
Transcription
mRNA
Translation
Inducer
(allolactose)
Repressor
protein
(inactive)
Transacetylase
Lactose
permease
β-galactosidase
Enzymes for lactose metabolism
Fig. 14-2b, p. 307
Repressible Genes (1)
•
A repressible operon (trp operon)
•
•
Repressor protein
•
•
•
is normally turned on
is synthesized in inactive form
cannot bind to operator
A metabolite (metabolic end product)
•
acts as corepressor
Repressible Genes (2)
•
With high intracellular corepressor levels
•
•
•
corepressor molecule binds to repressor
changes repressor’s shape
Altered repressor
•
•
binds to operator
turns off transcription of operon
The trp
Operon
Repressor
gene
trp operon
Promoter Operator trp E trp D trp C trp B trp A
DNA
mRNA
RNA
polymerase
Transcription
mRNA
Translation
Repressor
protein
(inactive)
Enzymes of the tryptophan
biosynthetic pathway
Tryptophan
(a) Intracellular tryptophan levels low.
Fig. 14-4a, p. 310
Repressor
gene
trp operon
Promoter Operator trp E
DNA
trp D trp C trp B trp A
Active repressor –
corepressor complex
mRNA
Inactive repressor
protein
Tryptophan
(corepressor)
(b) Intracellular tryptophan levels high.
Fig. 14-4b, p. 310
Constitutive Genes (1)
•
Are neither inducible nor repressible
•
•
active at all times
Regulatory proteins
•
produced constitutively
• catabolite activator protein (CAP)
• repressor proteins
Constitutive Genes (2)
•
Regulatory proteins
•
•
recognize and bind to specific base
sequences in DNA
Activity of constitutive genes
•
controlled by binding RNA polymerase to
promoter regions
Learning Objective 4
•
What is the difference between positive
and negative control?
•
How do both types of control operate in
regulating the lac operon?
Negative Control
•
Repressible and inducible operons are
under negative control
•
When repressor protein binds to operator
•
transcription of operon is turned off
Positive Control (1)
•
Some inducible operons are under positive
control
•
Activator protein binds to DNA
•
stimulates transcription of gene
Positive Control (2)
•
CAP activates lac operon
•
•
•
To bind to lac operon
•
•
binds to promoter region
stimulates transcription by tightly binding RNA
polymerase
CAP requires cyclic AMP (cAMP)
cAMP levels increase
•
as glucose levels decrease
Positive Control
Promoter
Repressor
gene
CAPRNA
binding polymerase –
site
binding site Operator lac Z lac Y lac A
DNA
mRNA
RNA polymerase
binds poorly
CAP
(inactive)
Allolactose
Repressor
protein (inactive)
(a) Lactose high, glucose high, cAMP low.
Fig. 14-5a, p. 311
Promoter
Repressor
gene
CAPRNA
binding polymerase –
site
binding site Operator lac Z lac Y lac A
DNA
RNA
Transcription
polymerase
binds efficiently
mRNA
CAP
mRNA
Translation
cAMP
Allolactose
Repressor
protein (inactive)
(b) Lactose high, glucose low, cAMP high.
Galactoside
transacetylase
Lactose permease
β -galactosidase
Enzymes for lactose metabolism
Fig. 14-5b, p. 311
Binding CAP
DNA
cAMP
CAP dimer
Fig. 14-6, p. 312
Learning Objective 5
•
What are the types of posttranscriptional
control in bacteria?
Posttranscriptional Controls
in Bacteria
•
Translational control
•
•
regulates translation rate of particular mRNA
Posttranslational controls
•
include feedback inhibition of key enzymes in
metabolic pathways
KEY CONCEPTS
•
Prokaryotes regulate gene expression in
response to environmental stimuli
KEY CONCEPTS
•
Gene regulation in prokaryotes occurs
primarily at the transcription level
Learning Objective 6
•
Discuss the structure of a typical
eukaryotic gene and the DNA sequences
involved in regulating that gene
Eukaryotic Genes
•
Are not normally organized into operons
•
Regulation occurs at levels of
•
•
•
•
Transcription
mRNA processing
Translation
Modifications of protein product
Transcription
•
Requires
•
Transcription initiation site
• where transcription begins
•
Promoter
• to which RNA polymerase binds
•
In multicellular eukaryotes
•
RNA polymerase binds to promoter (TATA
box)
Transcription
TATA box
UPE
T T
TATA A
A A
Transcription
initiation
site
pre-mRNA
(a) Eukaryotic promoter elements.
Fig. 14-9a, p. 316
TATA box
UPE
T T
TATA A
A A
Transcription
initiation
site
pre-mRNA
(b) A weak eukaryotic promoter.
Fig. 14-9b, p. 316
UPE
UPE
UPE
UPE
TATA box
T T
TATA A
A A
Transcription
initiation
site
pre-mRNA
(c) A strong eukaryotic promoter.
Fig. 14-9c, p. 316
Enhancer
UPE
UPE
TATA box
T T
TATA A
A A
Transcription
initiation
site
pre-mRNA
(d) A strong eukaryotic promoter plus an enhancer.
Fig. 14-9d, p. 316
Regulated Eukaryotic Gene
•
Promoter
•
•
•
RNA polymerase-binding site
short DNA sequences (upstream promoter
elements (UPEs) or proximal control elements)
UPEs
•
number and types within promoter region
determine efficiency of promoter
Enhancers (1)
•
Located far away from promoter
•
•
control some eukaryotic genes
Help form active transcription initiation
complex
Enhancers (2)
•
Specific regulatory proteins
•
•
bind to enhancer elements
activate transcription by interacting with
proteins bound to promoters
Enhancers
Enhancer
Target
proteins
RNA
polymerase
TATA box
DNA
(a) Little or no transcription.
Fig. 14-11a, p. 317
Enhancer
DNA
TATA box
Activator
(transcription factor)
(b) High rate of transcription.
Fig. 14-11b, p. 317
Learning Objective 7
•
In what ways may eukaryotic DNA-binding
proteins bind to DNA?
Transcription Factors
•
DNA-binding protein regulators control
eukaryotic genes
•
•
some transcriptional activators
some transcriptional repressors
Transcription Factors
•
Each has DNA-binding domain
•
3 types of regulatory proteins
•
•
•
Helix-turn-helix
Zinc fingers
Leucine zippers
Helix-Turn-Helix
•
Inserts one helix into DNA
Turn
α -helix
DNA
(a) Helix-turn-helix.
Fig. 14-10a, p. 317
Zinc Fingers
•
Loops of amino acids
•
•
held together by zinc ions
each loop has α-helix that fits into DNA
COO–
Finger 2
Finger 3
Zinc
ion
Finger 1
NH3+
DNA
(b) Zinc fingers.
Fig. 14-10b, p. 317
Leucine Zipper Proteins
•
Associate as dimers that insert into DNA
Leucine
zipper
region
DNA
(c) Leucine zipper.
Fig. 14-10c, p. 317
Learning Objective 8
•
How may a change in chromosome
structure affect the activity of a gene?
Gene Activity (1)
•
Changes in chromosome structure
•
•
inactivates genes
Heterochromatin
•
•
densely packed regions of chromosomes
contain inactive genes
Gene Activity (2)
•
Active genes
•
•
associated with loosely packed chromatin
structure (euchromatin)
Cells change chromatin structure
•
•
from heterochromatin to euchromatin
by chemically modifying histones (proteins
associated with DNA to form nucleosomes)
Chromatin Structure
Heterochromatin: genes
silent
Chromatin
decondensation
Nucleosome
Histones
DNA
Transcribed
region
Euchromatin: genes active
Fig. 14-7, p. 314
Gene Activity (3)
•
Histone tail
•
•
string of amino acids that extends from the
DNA-wrapped nucleosome
Methyl groups, acetyl groups, sugars, and
proteins
•
•
may chemically attach to the histone tail
may expose or hide genes (turn on or off)
Gene Activity (4)
•
Epigenetic inheritance
•
•
•
changes how a gene is expressed
important mechanism of gene regulation
DNA methylation
•
•
•
perpetuates gene inactivation
patterns repeat in successive cell generations
mechanism for epigenetic inheritance
Gene Amplification
•
Some genes
•
•
•
products are required in large amounts
have multiple copies in the chromosome
Gene amplification
•
some cells selectively amplify genes by DNA
replication
Gene Amplification
Drosophila chorion gene
Gene amplification by
repeated DNA replication
of chorion gene region
Chorion gene in ovarian cell
Fig. 14-8, p. 315
Learning Objective 9
•
How may a gene in a multicellular
organism produce different products in
different types of cells?
Differential mRNA Processing
•
Single gene produces different forms of
protein in different tissues
•
•
depending on how pre-mRNA is spliced
Gene contains a segment that can be
either intron or exon
•
•
as intron, sequence is removed
as exon, sequence is retained
Differential mRNA Processing
Potential splice sites
Exon
Intron
Exon
or intron
Exon
pre-mRNA
Differential
mRNA processing
Exon
Exon
Exon
Functional mRNA in
tissue A
Exon
Exon
Functional mRNA in
tissue B
Fig. 14-12, p. 318
Learning Objective 10
•
What types of regulatory controls operate
in eukaryotes after mature mRNA is
formed?
mRNA Stability
•
Certain regulatory mechanisms increase
RNA stability
•
•
allowing more protein synthesis before mRNA
degradation
Sometimes under hormonal control
Posttranslational Control (1)
•
In eukaryotic gene expression
•
•
•
feedback inhibition
modification of protein structure
Protein function change
•
•
by kinases adding phosphate groups
by phosphatases removing phosphates
Protein Degradation (1)
•
Proteins targeted for destruction
•
•
covalently bonded to ubiquitin
Protein tagged by ubiquitin
•
degraded in a proteasome
Protein Degradation (2)
•
Proteasome
•
•
•
large macromolecular structure
recognizes ubiquitin tags
Proteases
•
•
•
protein-degrading enzymes
associated with proteasomes
degrade protein into peptide fragments
Protein
Degradation
Target protein
Ubiquitin
1
Ubiquitin molecules
attach to protein targeted for degradation.
2
Protein enters
proteasome.
Ubiquitinylated
protein
Proteasome
3 Ubiquitins are
released and
available for
reuse. Protein
is degraded
into peptide
fragments.
Peptide
fragments
Fig. 14-13, p. 318
Target protein
Ubiquitin
1 Ubiquitin molecules
attach to protein targeted for degradation.
Ubiquitinylated
protein
2
Protein enters
proteasome.
Proteasome
3
Ubiquitins are
released and
available for
reuse. Protein
is degraded
into peptide
fragments.
Peptide
fragments
Stepped Art
Fig. 14-13, p. 318
KEY CONCEPTS
•
Gene regulation in eukaryotes occurs at
the levels of transcription,
posttranscription, translation, and
posttranslation
Animation: Controls of
Eukaryotic Gene Expression
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