Download Figure 10-14: Cooperative binding of activators.

Li Xiaoling
Office:
QQ:
M1623
313320773
E-MAIL: 313320773 @qq.com
2017/5/6
Content
Chapter 1 Introduction
Chapter 2 The Structures of DNA and RNA
Chapter 3 DNA Replication
Chapter 4 DNA Mutation and Repair
Chapter 5 RNA Transcription
Chapter 6 RNA Splicing
Chapter 7 Translation
Chapter 8 The Genetic code
Chapter 9 Regulation in prokaryotes
Chapter 10 Regulation in Eukaryotes
2017/5/6
HOW TO LEARN THIS COURSE WELL？
 To learn effectively
To preview and review
Problem-base learning
Making use of class time effectively
 Active participation
Bi-directional question in class
Group discussion
Concept map
 Tutorship
 To call for reading, thingking and discussing of investigative learning
EVALUATION (GRADING) SYSTEM
 Question in-class and attendance : 10 points
 Group study and attendance: 20 points
 Final exam: 70 points
 Bonus
Molecular Biology of the Gene,
5/E --- Watson et al. (2004)
Part I: Chemistry and Genetics
Part II: Maintenance of the Genome
Part III: Expression of the Genome
Part IV: Regulation
Surfing the contents of Part IV
--The heart of the frontier
biological disciplines
6
•Molecular Biology Course
Chapter 10
Gene Regulation
in Eukaryotes
7
TOPIC 1 Conserved Mechanisms of
Transcriptional Regulation from Yeast to
Human.
TOPIC 2 Recruitment of Protein Complexes to
Genes by Eukaryotic Activators.
TOPIC 3 Transcriptional Repressors
TOPIC 4 Signal Integration and Combinatorial
Control.
TOPIC 5 Signal Transduction and the Control of
Transcriptional Regulators.
TOPIC 6 Gene Silencing by Modification of
Histones and DNA.
TOPIC 7 Epigenetic Gene Regulation.
8
Principles of Transcription Regulation
1. Gene Expression is Controlled
by Regulatory Proteins (调控蛋白)
Gene expression is very often
controlled by Extracellular
Signals, which are communicated
to genes by regulatory proteins:
 Positive regulators or activators
INCREASE the transcription
 Negative regulators or repressors
DECREASE or ELIMINATE the
transcription
9
Similarity of regulation between
eukaryotes and prokaryote
1.Principles are the same:
•
signals (信号),
•
activators and repressors (激活蛋白和阻
遏蛋白)
•
recruitment and allostery, cooperative
binding (招募，异构和协同结合)
2. The gene expression steps subjected to
regulation are similar, and the initiation
of transcription is the most pervasively
regulated step.
10
Difference in regulation between eukaryotes
and prokaryote
1.
2.
3.
4.
Pre-mRNA splicing adds an important step
for regulation. (mRNA前体的剪接)
The eukaryotic transcriptional machinery is
more elaborate than its bacterial
counterpart. (真核转录机器更复杂)
Nucleosomes and their modifiers influence
access to genes. (核小体及其修饰体)
Many eukaryotic genes have more
regulatory binding sites and are controlled
by more regulatory proteins than are
bacterial genes. (真核基因有更多结合位点)
11
A lot more regulator bindings sites in
multicellular organisms reflects the
more extensive signal integration
Bacteria
Yeast
Human
12
Fig. 10-1
Enhancer (激活元件) : a given site binds
regulator responsible for activating the
gene. Alternative enhancer binds different
groups of regulators and control expression
of the same gene at different times and
places in responsible to different signals.
Activation at a distance is much more
common in eukaryotes.
Insulators (绝缘子) or boundary elements (临
界元件) are regulatory sequences between
enhancers and promoters. They block
activation of a linked promoter by activator
bound at the enhancer, and therefore
ensure activators work discriminately.
13
CHAPTER 10 Gene Regulation in eukaryotes
一、真核的转录激活蛋白的结构特征
The structure features of the
eukaryotic transcription activators
Topic 1: Conserved
Mechanisms of
Transcriptional Regulation
from Yeast (酵母) to
Mammals (哺乳动物)
14
The basic features of gene regulation are
the same in all eukaryotes, because of
the similarity in their transcription and
nucleosome structure.
Yeast is the most amenable to both
genetic and biochemical dissection, and
produces much of knowledge of the action
of the eukaryotic repressor and activator.
The typical eukaryotic activators works in
a manner similar to the simplest bacterial
case.
Repressors work in a variety of ways.
15
1. Eukaryotic activators (真核激活蛋白) have
separate DNA binding and activating
functions【与原核相似】, which are very
often on separate domains of the protein.
Fig. 10-2 Gal4 bound to its site on DNA
16
Eukaryotic activators---Example 1: Gal4
 Gal4 is the most studied eukaryotic activator
 Gal4 activates transcription of the galactose
genes in the yeast S. cerevisae.
 Gal4 binds to four sites (UASG) upstream of
GAL1, and activates transcription 1,000-fold
in the presence of galactose
17
Fig. 10-3 The regulatory sequences of the Yeast GAL1 gene.
Eukaryotic activators---Example 1: Gal4
Experimental evidences showing that Gal4
contains separate DNA binding and activating
domains.
1. Expression of the N-terminal region (DNAbinding domain) of the activator produces a
protein bound to the DNA normally but did
not activate transcription.
2. Fusion of the C-terminal region (activation
domain) of the activator to the DNA binding
domain of a bacterial repressor, LexA
activates the transcription of the reporter
gene. Domain swap experiment
实验介绍系列1－Experiment introduction series
18
Domain swap
experiment
Moving domains
among proteins,
proving that domains
can be dissected
into separate parts
of the proteins.
Many similar
experiments
shows that DNA
binding domains
and activating
regions are
19
separable.
Box 1 The two hybrid Assay (酵母双杂交) is
used to identify proteins interacting with
each other. （实验介绍系列2）
Fuse protein A and protein B
genes to the DNA binding
domain and activating region of
Gal4, respectively.
Produce fusion proteins
20
2. Eukaryotic regulators use a range of DNA
binding domains, but DNA recognition
involves the same principles as found in
bacteria.
 Homeodomain proteins
 Zinc containing DNA-binding domain:
zinc finger and zinc cluster
 Leucine zipper motif
 Helix-Loop-Helix proteins : basic zipper
and HLH proteins
21
Bacterial regulatory proteins
• Most use the helix-turn-helix motif to
bind DNA target
• Most bind as dimers to DNA sequence:
each monomer inserts an a helix into
the major groove.
Eukaryotic regulatory proteins
1. Recognize the DNA using the similar
principles, with some variations in detail.
2. In addition to form homodimers (同源二聚
体), some form heterodimers (异源二聚体)
to recognize DNA, extending the range
of DNA-binding specificity.
22
Homeodomain proteins: The homeodomain
(同源结构域) is a class of helix-turn-helix
DNA-binding domain and recognizes DNA
in essentially the same way as those
bacterial proteins.
What is the same?
Figure 10-5
23
Zinc containing DNA-binding domains (锌指
结构域): Zinc finger proteins (TFIIIA) and
Zinc cluster domain (Gal4)
Figure 10-6
24
Leucine Zipper Motif (亮氨酸拉链基序) : The
Motif combines dimerization and DNAbinding surfaces within a single structural
unit.
Figure 10-7
25
Dimerization (二聚化) is mediated by
hydrophobic interactions between
the appropriately-spaced leucine
(亮氨酸) to form a coiled coil
structure
26
27
Helix-Loop-Helix motif: similar as
in leucine zipper motif.
Figure 10-8
28
myogenic factor:生肌调节蛋白是
一种转录因子。
29
Because the region of the a-helix
that binds DNA contains baisc
amino acids residues, Leucine
zipper and HLH proteins are often
called basic zipper and basic HLH
proteins.
Both of these proteins use
hydrophobic amino acid residues
for dimerization.
30
3. Activating regions (激活区域)
are not well-defined structures
The activating regions are grouped on
the basis of amino acids content.
 Acidic activation region (酸性激活区域):
contain both critical acidic amino acids
and hydrophobic acids.yeast Gal4
 Glutamine-rich region (谷氨酰胺富集区):
mammalian activator SP1
 Proline-rich region (脯氨酸富集区):
mammalian activator CTF1
31
CHAPTER 17 Gene Regulation in eukaryotes
二、真核转录激活蛋白的招募调控方式和远距调控特征
Activation of the eukaryotic transcription
by recruitment & Activation at a distance
Topic 2: Recruitment of
Protein Complexes
to Genes by
Eukaryotic Activators
32
Eukaryotic activators (真核激活蛋白)
also work by recruiting (招募) as in
bacteria, but recruit polymerase
indirectly in two ways:
1. Interacting with parts of the
transcription machinery.
2. Recruiting nucleosome modifiers
that alter chromatin in the vicinity
of a gene.
33
1. Activators recruit the transcription
machinery to the gene.
34
The eukaryotic transcriptional machinery contains
polymerase and numerous proteins being
organized to several complexes, such as the
Mediator and the TFⅡD complex. Activators
interact with one or more of these complexes
and recruit them to the gene.
Figure 10-9
35
Box 2 Chromatin Immuno-precipitation (ChIP) (染
色质免疫共沉淀) to visualize where a given protein
(activator) is bound in the genome of a living cell.)
（实验介绍系列3）
36
Activator Bypass Experiment (越过激活子实验)Activation of transcription through direct
tethering of mediator to DNA. （实验介绍系列4）
Directly fuse the
bacterial DNAbinding protein
LexA protein to
Gal11, a component
of the mediator
complex to activate
GAL1 expression.
Figure 10-1037
At most genes, the transcription
machinery is not prebound, and
appear at the promoter only upon
activation. Thus, no allosteric
activation of the prebound
polymerase has been evident in
eukaryotic regulation。
38
2. Activators also recruit modifiers that
help the transcription machinery bind
at the promoter
Two types of Nucleosome modifiers :
Those add chemical groups to the tails
of histones (在组蛋白尾上加化学基团), such
as histone acetyl transferases (HATs)
Those remodel the nucleosomes (重塑核
小体), such as the ATP-dependent
activity of SWI/SNF.
39
How the nucleosome modification
help activate a gene?
1.
“Loosen” the chromatin structure by
chromosome remodeling (Fig. 10-11b)
and histone modification such as
acetylation (Fig. 10-11a), which
uncover DNA-binding sites that
would otherwise remain inaccessible
within the nucleosome.
40
uncover DNA-binding sites
（组蛋白乙酰化酶）
Fig 10-11 Local alterations in chromatin
41
2. Adding acetyl groups to histones
helps the binding of the
transcriptional machinery. One
component of TFIID complex bears
bromodomains that specifically bind
to the acetyl groups. Therefore, a
gene bearing acetylated nucleosomes
at its promoter have a higher
affinity for the transcriptional
machinery than the one with
unacetylated nucleosomes.
42
溴区结构域蛋白
One component
of TFIID
complex bears
bromodomains.
Figure 10-39 Effect of histone tail
43
3. Action at a distance: loops and
insulators
Many enkaryotic activators－particularly
in higher eukaryotes－work from a distance.
How?
1. Some proteins help, for example Chip
protein in Drosophila.
2. The compacted chromosome structure help.
DNA is wrapped in nucleosomes in
eukaryotes. So sites separated by many
base pairs may not be as far apart in the
cell as thought.
44
Specific cis-acting elements
called insulators (绝缘子) control
the actions of activators,
preventing the activating the
non-specific genes
45
Insulators
block
activation
by
enhancers
Figure 10-12
46
Transcriptional Silencing (转录沉默)
Transcriptional Silencing is a specialized form of
repression that can spread along chromatin,
switching off multiple genes without the need
for each to bear binding sites for specific
repressor.
Insulator elements (绝缘子元件) can block this
spreading, so insulators protect genes from
both indiscriminate activation and
repression.[绝缘子的概念到此才介绍完毕]
Application：A gene inserted at random into the
mammalian genome is often “silenced”, and
placing insulators upstream and downstream of
that gene can protect the gene from silencing.
47
4 Appropriate regulation of some groups of
genes requires locus control region (LCR).
1.
2.
Human and mouse globin genes are clustered
in genome and differently expressed at
different stages of development
A group of regulatory elements collectively
called the locus control region (LCR), is
found 30-50 kb upstream of the cluster of
globin genes. It binds regulatory proteins
that cause the chromatin structure to “open
up”, allowing access to the array of
regulators that control expression of the
individual genes in a defined order.
48
Figure 10-13
Please compare LCR with the Lac operon
controlled gene expression in bacteria
49
Another group of mouse genes whose
expression is regulated in a temporarily
and spatially ordered sequence are
called HoxD genes. They are controlled
by an element called the GCR (global
control region) in a manner very like
that of LCR.
50
CHAPTER 10 Gene Regulation in eukaryotes
三、真核转录阻遏蛋白（或抑制蛋白）及其调控
Topic 3: Transcriptional
Repressor & its regulation
In eukaryotes, most repressors do not
repress transcription by binding to sites
that overlap with the promoter and thus
block binding of polymerase. (Bacteria
often do so)
51
Commonly, eukaryotic repressors recruit
nucleosome modifiers that compact the
nucleosome or remove the groups
recognized by the transcriptional
machinery [contrast to the activator
recruited nucleosome modifers, histone
deacetylases (组蛋白去乙酰化酶) removing
the acetyl groups]. Some modifier adds
methyl groups to the histone tails, which
frequently repress the transcription.
This modification causes transcriptional
silencing.
52
Three other ways in which an eukaryotic
repressor works include:
(1) Competes with the activator for an
overlapped binding site.
(2) Binds to a site different from that of
the activator, but physically interacts
with an activator and thus block its
activating region.
(3) Binds to a site upstream of the
promoter, physically interacts with the
transcription machinery at the promoter
to inhibit transcription initiation.
53
Competes for the
activator binding
Inhibits the function
of the activator.
Figure 10-19： Ways in which eukaryotic
repressor work
54
Binds to the
transcription
machinery
Recruits nucleosome
modifiers (most common)
55
A specific example: Repression of
the GAL1 gene in yeast
In the presence of glucose, Mig1 binds to a
site between the USAG and the GAL1 promoter,
and recruits the Tup1 repressing complex.
Tup1 recruits histone deacetylases, and also
directly interacts with the transcription
machinery to repress transcription.
56
CHAPTER 10 Gene Regulation in eukaryotes
四、真核转录调控的特色：信号整合和组合调控
Features of the eukaryotic transcriptional
regulation: signal integration and
combinatorial control
Topic 4: Signal Integration
and Combinatorial
Control
57
1. Activators work together
synergistically (协作地) to
integrate signals.
激活蛋白一起协作来整合多种信号
58
Review the Lac operon control in bacteria. Two
signals are integrated to control Lac expression
Glucose
Lactose
59
Figure 10-6
In multicellular organisms, signal
integration (信号整合) is used
extensively. In some cases, numerous
signals are required to switch a gene on.
However, each signal is transmitted to
the gene by a separate regulator, and
therefore, multiple activators often
work together, and they do so
synergistically (two activators working
together is greater than the sum of
each of them working alone.)
60
Three strategies of the synergy (协作
的三种方式):
S1: Multiple activators recruit a single
component of the transcriptional machinery.
For example, by touching the different part of
the mediator complex (中介体复合物). The
combined binding energy has an exponential
effect on recruitment.
S2: Multiple activators each recruit a
different component of the transcriptional
machinery. These components binds to the
promoter DNA inefficiently without help.
61
S3: Multiple activators help each other
bind to their sites upstream of the gene
they control. (Figure 10-14)
62
Figure 17-14:
Cooperative binding
of activators
a.“Classical”
cooperative binding.
b. Both proteins
interacting with a
third protein.
c. The first protein
recruit a nucleosome
remodeller whose
action reveal a
binding site for the
second protein.
d. Binding a protein
unwinds the DNA
from nucleosome a
little, revealing the
binding site for
another protein.
63
2. Signal integration: the HO gene is
controlled by two regulators; one
recruits nucleosome modifiers and the
other recruits mediator. [S3策略的
example]
The HO gene is only expressed in mother
cells and only a certain point in the cell
cycle, resulting in the budding division
feature of yeast S. cerevisiae (啤酒酵母).
The mother cell and cell cycle conditions
(signals) are communicated to the HO gene
(target) by two activators: SWI5 and SBF
(communicators).
64
SWI5: acts only in the mother cell and binds to
multiple sites some distance from the gene unaided,
which recruit enzymes to open the SBF binding sites.
SBF: only active at the correct stages of the cell
cycle, and cannot bind the sites unaided.
Alter the nucleosome
(组蛋白
乙酰化酶)
Figure 10-15
65
Figure 10-14: Cooperative binding of
activators. (c) The first protein recruit a
nucleosome remodeller whose action reveal a
binding site for the second protein.
66
3. Signal integration: Cooperative
binding of activators at the human binterferon gene. [S3策略的example]
The human b-interferon gene (target gene)
is activated in cells upon viral infection
(signal). Infection triggers three activators
(communicator): NFkB, IRF, and Jun/ATF.
Activators bind cooperatively to sites
adjacent to one another within an enhancer
located about 1 kb upstream of the promoter,
which forms a structure called enhanceosome.
67
(人的b干扰素基因)
增强子
增强体
1. Activators interact with each
other
2. HMG-I binds within the
enhancer and aids the binding
of the activators (bends the
DNA to promote the
activator interaction)
Figure 10-16
68
HMG-I is constitutively active in the
cells, and play an architectural
role in the INF-b gene activation
process.
69
Figure 10-14: Cooperative binding of
activators. (a)“Classical” cooperative binding
through direct interaction between the two
proteins.
70
4. Combinatory control (组合调控) lies
at the heart of the complexity and
diversity of eukaryotes, in which Both
activators and repressors work together.
组合调控是真核生物复杂性和多样性的核心。
它属于协作调控的一种具体方式。在组合调
控中，激活蛋白和阻遏蛋白共同起作用。
71
Review Page 499 & Slide 47 in Ch 16
7: Combinatorial Control (组合调
控): CAP controls other genes as
well


A regulator (CAP) works together with
different repressors at different genes,
this is an example of Combinatorial
Control.
In fact, CAP acts at more than 100
genes in E.coli, working with an array of
partners.
72
There is extensive combinatorial control
in eukaryotes.---A generic picture
Four
signals
Three
signals
Figure 10-18
In complex multicellular organisms, combinatorial
control involves many more regulators and genes
than shown above. Both activators and
repressors can be involved.
73
5. An example: combinatory
control of the mating-type genes
from S. cerevisiae (啤酒酵母的交配
型的组合调控)
74
The yeast S. cerevisiae exists in
three forms:
---two haploid cells (单倍体) of
different mating types－ a and a.
---the diploid cells (双倍体) (a/a)
formed when an a and an a cell mate
and fuse.
Cells of the two mating types (a and
a) differ because they express
different sets of genes: a specific
genes and a specific genes.
75
a cells make the regulatory protein a1,
a cells make the protein a1 and a2.
Both cell types express the fourth
regulator protein Mcm1 that is also
involved in regulatory the mating-type
specific genes.
How do these regulators work together
to keep a cell in its own type? [Figure
17-19]
76
Figure 17-19: Control of cell-type specific genes
in yeast
Cooperative binding
Cooperative binding
77
CHAPTER 10 Gene Regulation in eukaryotes
五、基于真核转录调控的前沿学科：信号传导
Signal transduction---A life science
frontier centered on the eukaryotic
transcriptional regulation.
Topic 5: Signal
Transduction (信号传导) and
the Control of
Transcriptional Regulators
78
Topic 4: Signal Transduction and the Control of Transcriptional Regulators
1. Signals are often communicated to
transcriptional regulators through
signal transduction pathway
信号经常通过信号传导途径被运输到转录调节蛋白
79
Environmental Signals/Information (信号)
1. Small molecules such as sugar,
histamine (组胺).
2. Proteins released by one cell and
received by another.
In eukaryotic cells, most signals are
communicated to genes through signal
transduction pathway (indirect), in which
the initiating ligand is detected by a
specific cell surface receptor.
What about in bacteria?
80
Signal transduction pathway
1. The initial ligand (“signal”) binds to an
extracellular domain of a specific cell
surface receptor
2. The signal is thus communicated to
the intracellular domain of receptor (via
an allosteric change or dimerization )
3*. The signal is then relayed (分程传递) to
the relevant transcriptional regulator.
4. The transcriptional regulator control
the target gene expression (topic 2-4).
81
a. The STAT pathway
b. The MAP kinase pathway
Figure 10-22：Signal
transduction pathway
82
Topic 4: Signal Transduction and the Control of Transcriptional Regulators
2. Signals control the activities of
eukaryotic transcriptional regulators
in a variety of ways---The
mechanisms of signal transduction.
信号传导的机制：信号经常通过不同方式控制转
录调节蛋白的活性
In eukaryotes, a signal can be
communicated, directly or indirectly,
to a transcriptional regulator.
83
Mechanism 1: unmasking an activating
region (Topic 2 & 3):
(1)
(2)
(3)
A conformational change to reveal the
previously buried activating region.
Releasing of the previously bound masking
protein. Example: the activator Gal4 is
controlled by the masking Gal80) (Fig.17-23).
Some masking proteins not only block the
activating region of an activator but also
recruit a deacetylase enzyme to repress the
target genes. Example: Rb represses the
function of the mammalian transcription
activator E2F in this way. Phosphorylation of
Rb releases E2F to activate the target gene
expression.
84
Activator Gal4 is regulated by a
masking protein Gal80
Gal4
Figure 10-23
85
Mechanism 2: Transport into and out of
the nucleus (Fig.10-21) :
When not active, many activators and
repressors are held in the cytoplasm. The
signaling ligand causes them to move into the
nucleus where they activate transcription
(Fig.10-4b).
86
Other Mechanisms #1: A cascade of
kinases that ultimately cause the
phosphorylation of regulator in nucleus
(new) (Fig.19-4a).
87
Other Mechanisms #2: The activated
receptor is cleaved by cellular proteases
(蛋白酶), and the c-terminal portion of
the receptor enters the nuclease and
activates the regulator (new):(Fig.10-4c).
88
CHAPTER 10 Gene Regulation in eukaryotes
Topic 6: Gene “Silencing”
by Modification of
Histones and DNA
890
Transcriptional silencing is a position
effect. (1) A gene is silenced because
of where it is located, not in
response to a specific environmental
signal. (2) Silencing can spread over
large stretches of DNA, switching
off multiple genes, even those quite
distant from the initiating event.
The most common form of silencing is
associated with a dense form of
chromatin called “heterochromatin”.
Heterochromatin is frequently
associated with particular regions of
the chromosome, notably the
telomeres, and the centromeres.
In mammalian cells, about 50% of the
genome is estimated to be in some
form of heterochromatin.
Transcriptional silencing is associated
with Modification of nucleosomes that
alters the accessibility of a gene to
the transcriptional machinery and
other regulatory proteins.
The modification enzymes for silencing
include deacetylases, DNA
methylases.
Topic 6: Gene “Silencing” by Modification of Histones and DNA
6-1. Silencing in yeast is mediated by
deacetylation and methylation of the
histones
在酵母中的沉默是通过对组蛋白的
去乙酰化和甲基化介导的
93
The telomeres, the silent mating-type locus,
and the rDNA genes are all “silent” regions in
S. cerevisiae.
Three genes encoding regulators of silencing,
SIR2, 3, and 4 have been found (SIR stands
for Silent Information Regulator).
Rap1 recruits Sir
complex to the
temomere.
Sir2 deacetylates
nearby nucleosome.
Fig. 10-24. Silencing at the yeast telomere
Silencing specificity is determined by
Rap1, the telomere DNA-binding protein.
It can also be determined by RNA
molecules using RNAi machinery (Chapter
18).
The spreading of silencing is
restricted/controlled by insulators and
other kind of histone modifications that
block binding of the Sir2 proteins.
95
Transcription can also be silenced by
methylation of DNA by histone
methyltransferase (H3 and H4,
Chapter 7).
This enzyme have been recently found in
yeast, but is common in mammalian
cells. Its function is better understood
in higher eukaryotes.
In higher eukaryotes, silencing is
typically associated with chromatin
containing histones that both
deacetylated and methylated.
Topic 6: Gene “Silencing” by Modification of Histones and DNA
6-2. In Drosophila, HP1 recognizes
Methylated Histones and Condense
Chromatin.
在果蝇中，HP1蛋白识别甲基化的
组蛋白和浓缩的染色质。
97
HP1 protein is a component of
heterochromatin in Drosophila that
binds to methylated residue in histone
H3.
Such a histone modification is produced
by Su(Var)3-9.
Different types of modification at the
histones can be involved in distinct
geene regulation. What will happen
when multiple forms of modification
occur? ---A “Histone Code”
hypothesis.
Box 17-4 Is there a histone code?
According to this idea, different
patterns of modification on histone tails
can be “read” to mean different things.
The “meaning” would be the result of
the direct effects of these
modifications on chromatin density and
form.
But in addition, the particular pattern
of modifications at any given location
would recruit specific proteins.
Topic 6: Gene “Silencing” by Modification of Histones and DNA
6-3. DNA Methylation Is Associated
with Silenced Genes in Mammlian
cells.
DNA甲基化与哺乳动物细胞的沉默
基因相关联。【注DNA甲基化不是
组蛋白甲基化】
Some mammalian genes are kept silent by
methylation of nearby DNA sequences.
100
Large regions of mammalian genome are
marked by methylation of DNA
sequences, which is often seen in
heterochromatic regions. [Why?]
The methylated DNA sequences are often
recognized by DNA-binding proteins (such
as MeCP2) that recruit histone
decetylases and histone methylases,
which then modify nearby chromatin.
Thus, methylation of DNA can mark sites
where heterochromatin subsequently
forms (Fig. 17-25).
Fig. 10-25 Switching a gene off through DNA
methylation and the subsequent histone modification
DNA methylation lies at the heart of
Imprinting
Imprinting- in a diploid cell, one copy of
a gene from the father or mother is
expressed while the other copy is
silenced.
Two well-studied examples: human H19
and insulin-like growth factor 2 (Igf2)
genes.
Enhancer: activate both gene transcription
ICR: an insulator binds CTCF protein and blocks the
activity of the enhancer on Igf2.Methylation of ICR
allows the enhancer to activate Igf2.
H19 repression is mediated by DNA methylation and the
subsequent MeCP2 binding to the methylated ICR
Figure 10-26 Imprinting
104
CHAPTER 10 Gene Regulation in eukaryotes
Topic 7: Epigenetic
Regulation
105
Patterns of gene expression must
sometimes be inherited.
After the expression of specific genes in a
set of cells are switched on by a signal,
these genes may have to remain switched on
for many cell generations, even if the signal
that induced them is present only fleetingly
(飞快地).
The inheritance of gene expression
patterns, in the absence of both
mutation and the initiating signal, is
called epigenetic regulation.
Topic 7: Epigenetic Regulation
7-1. Some States of Gene Expression
Are Inherited through Cell Division
Even When the Initiating Signal Is No
Longer Present
有些基因表达状态在起始信号不存
在时仍然在细胞分裂中遗传。
The maintenance of a bacteriophage l
lysogen is an example of epigenetic
regulation. [Figure 17-27]
107
Establishment of lysogeny: synthesis of the
essential lysogenic l repressor is established by
transcription from one promoter and then
maintained by transcription from another one.
DNA methylation provides a mechanism of
epigenetic regulation. DNA methylation
is reliably inherited throughout cell
division [Fig. 17-28].
Certain DNA methylases can methylate,
at low frequency, previously unmodified
DNA; but far more efficiently, the socalled maintenance methylases modify
hemimethylated DNA－the very
substrate provided by replication of
fully methylated DNA.
[A link with reprogramming of somatic cells into
Embryonic Stem-like cells. Box 17-6]
Figure 10-28 Patterns of DNA methylation can
be maintained through cell division
Key points of the chapter
1. The structure features of the eukaryotic
transcription activators.
2. Activation of the eukaryotic transcription by
recruitment & Activation at a distance.
3. Transcriptional repressor & its regulation
4. Signal integration and combinatorial control
5. Signal transduction: communicating the
signals to transcriptional regulators.
6. Gene silencing and Epigenetic regulation.
7. 4 experimental methods introduced (yeast
two-hybrid and ChIP).
111