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41 103 Enhancers and Silencers

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41 103 Enhancers and Silencers
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10.3 Enhancers and Silencers
box, looks more like a class II promoter. Paradoxically,
removal of that TATA box converts the promoter from
class III to class II. Similarly, adding a TATA box to a U1 or
U2 snRNA promoter converts it from class II to class III.
One might have predicted just the opposite. By contrast, in
Drosophila and in sea urchins, some snRNA genes have
TATA boxes and others do not, but other sequence elements,
not the TATA boxes, determine whether the promoters are
class II or class III.
depressed transcription in vivo. This behavior suggested
that the 72-bp repeats constituted another upstream promoter element. However, Paul Berg and his colleagues discovered that the 72-bp repeats still stimulated transcription
even if they were inverted or moved all the way around to
the opposite side of the circular SV40 genome, over 2 kb
away from the promoter. The latter behavior, at least, is very
un-promoter-like. Thus, such orientation- and positionindependent DNA elements are called enhancers to
distinguish them from promoter elements.
How do enhancers stimulate transcription? We will see
in Chapter 12 that enhancers act through proteins that
bind to them. These have several names: transcription
factors, enhancer-binding proteins, or activators. These proteins appear to stimulate transcription by interacting with
other proteins called general transcription factors at the
promoter. This interaction promotes formation of a preinitiation complex, which is necessary for transcription. Thus,
enhancers usually allow a gene to be induced (or sometimes repressed) by activators. We will discuss these interactions in much greater detail in Chapters 11 and 12 and
we will see that activators frequently require help from
other molecules (e.g., hormones and coactivator proteins)
to exert their effects.
We frequently find enhancers upstream of the promoters they control, but this is by no means an absolute rule. In
fact, as early as 1983 Susumo Tonegawa and his colleagues
found an example of an enhancer within a gene. These investigators were studying a gene that encodes the larger
subunit of a particular mouse antibody, or immunoglobulin, called g2b. They introduced this gene into mouse plasmacytoma cells that normally expressed antibody genes,
but not this particular gene. To detect efficiency of expression of the transfected cells, they added a labeled amino
acid to tag newly made proteins, then immunoprecipitated
the labeled g2b protein (Chapter 5) with an antibody directed against g2b. Then they electrophoresed the immunoprecipitated proteins and detected them by autoradiography.
The suspected enhancer lay in one of the gene’s introns, a
region within the gene that is transcribed, but is subsequently cut out of the transcript by a process called splicing
(Chapter 14). Tonegawa and colleagues began by deleting
two chunks of DNA from this suspected enhancer region,
as shown in Figure 10.27a. Then they assayed for expression of the g2b gene in cells transfected by this mutated
DNA. Figure 10.27b shows the results: The deletions
within the intron, though they should have no effect on the
protein product because they are in a noncoding region of
SUMMARY At least one class III gene, the 7SL
RNA gene, contains a weak internal promoter, as
well as a sequence in the 59-flanking region of the
gene that is required for high-level transcription.
Other nonclassical class III genes (e.g., 7SK, and
U6 RNA genes) lack internal promoters altogether,
and contain promoters that strongly resemble class
II promoters in that they lie in the 59-flanking
region and contain TATA boxes. The U1 and U6
snRNA genes have nonclassical class II and III promoters, respectively. The U1 snRNA promoter has
an essential proximal sequence element (PSE), and
a distal sequence element (DSE) and is transcribed
by polymerase II. The U6 snRNA promoter has a
PSE, a DSE, and a TATA box, and is transcribed by
polymerase III.
10.3 Enhancers and Silencers
Many eukaryotic genes, especially class II genes, are associated with cis-acting DNA elements that are not strictly part
of the promoter, yet strongly influence transcription. As we
learned in Chapter 9, enhancers are elements that stimulate
transcription. Silencers, by contrast, depress transcription.
We will discuss these elements briefly here and expand on
their modes of action in Chapters 12 and 13.
Enhancers
Chambon and colleagues discovered the first enhancer in
the 59-flanking region of the SV40 early gene. This DNA
region had been noticed before because it contains a conspicuous duplication of a 72-bp sequence, called the 72-bp
repeat (Figure 10.26). When Benoist and Chambon made
deletion mutations in this region, they observed profoundly
72 bp
72 bp
GC
267
GC
GC
GC
GC
GC
TATA
Figure 10.26 Structure of the SV40 virus early control region. As usual, an arrow with a right-angle bend denotes the transcription initiation site,
although this is actually a cluster of three sites, as we saw in Figure 10.19. Upstream of the start site we have, in right-to-left order, the TATA box
(red), six GC boxes (yellow), and the enhancer (72-bp repeats, blue).
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Chapter 10 / Eukaryotic RNA Polymerases and Their Promoters
(a)
X2
X3
X4
X2
X3
Δ1 (180 bp)
Δ2 (470 bp)
1 2 3 4 5 6 7 8 9 1011 12 13
(b)
(c)
1
2
C+
C–
3
4
5
6
7
8
γ2b–
3.4 –
λ–
1.7 –
C
γ2bw.t.
Δ1
Δ2
w.t.
Δ1
Δ2
Figure 10.27 Effects of deletions in the immunoglobulin g2b H-chain
enhancer. (a) Map of the cloned g2b gene. The blue boxes represent the
exons of the gene, the parts that are included in the mRNA that comes
from this gene. The lines in between boxes are introns, regions of the
gene that are transcribed, but then cut out of the mRNA precursor as it is
processed to the mature mRNA. X2, X3, and X4 represent cutting sites for
the restriction enzyme XbaI. Tonegawa and colleagues suspected an
enhancer lay in the X2–X3 region, so they made deletions D1 and D2 as
indicated by the red boxes. (b) Assay of expression of the g2b gene at
the protein level. Tonegawa and colleagues transfected plasmacytoma
cells with the wild-type gene (lanes 2–5), the gene with deletion D1 (lanes
6–9), or the gene with deletion D2 (lanes 10–13). Lane 1 was a control
with untransfected plasmacytoma cells. After transfecting the cells,
these investigators added a radioactive amino acid to label any newly
made protein, then extracted the protein, immunoprecipitated the g2b
protein, electrophoresed the precipitated protein, and detected the
radioactive protein by fluorography (a modified version of
autoradiography in which a compound called a fluor is added to the
electrophoresis gel). Radioactive emissions excite this fluor to give off
photons that are detected by x-ray film. The D1 deletion produced
only a slight reduction in expression of the gene, but the D2 deletion
gave a profound reduction. (c) Assay of transcription of the g2b gene.
Tonegawa and colleagues electrophoresed and Northern blotted
RNA from the following cells: lane 1 (positive control), untransfected
plasmacytoma cells (MOPC 141) that expressed the g2b gene; lane 2
(negative control), untransfected plasmacytoma cells (J558L) that did not
express the g2b gene; lanes 3 and 4, J558L cells transfected with the
wild-type g2b gene; lanes 5 and 6, J558L cells transfected with the gene
with the D1 deletion; lanes 7 and 8, J558L cells transfected with the
gene with the D2 deletion. The D1 deletion decreased transcription
somewhat, but the D2 deletion abolished transcription. (Source: (b–c)
the gene, caused a decrease in the amount of gene product
made. This was especially pronounced in the case of the
larger deletion (D2).
Is this effect due to decreased transcription, or some
other cause? Tonegawa’s group answered this question by
performing Northern blots (Chapter 5) with RNA from cells
transfected with normal and deleted g2b genes. These blots,
shown in Figure 10.27c, again demonstrated a profound loss
of function when the suspected enhancer was deleted. But is
this really an enhancer? If so, one should be able to move it
or invert it and it should retain its activity. Tonegawa and
colleagues did this by first inverting the X2–X3 fragment, which
contained the enhancer, as shown in position/orientation B of
Figure 10.28a. Figure 10.28b shows that the enhancer still
functioned. Next, they took fragment X2–X3 out of the
intron and placed it upstream of the promoter (position/
orientation C). It still worked. Then they inverted it in its
new location (position/orientation D). Still it functioned.
Thus, some region within the X2–X3 fragment behaved as an
enhancer: It stimulated transcription from a nearby promoter, and it was position- and orientation-independent.
Finally, these workers compared the expression of this
gene when it was transfected into two different types of
mouse cells: plasmacytoma cells as before, and fibroblasts.
Gillies, S.D., S.L. Morrison, V.T. Oi, and S. Tonegawa, A tissue-specific transcription
enhancer element is located in the major intron of a rearranged immunoglobulin heavy
chain gene. Cell 33 (July 1983) p. 719, f. 2&3.)
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10.3 Enhancers and Silencers
(a)
D
C
X3
X2
X2
B
A
X3
X3
X2
(b)
A
B
C
269
D
1 2 3 4 5 6 7 8 9 10 11 12 M
X2
X3
γ2b
X1
X2/4
λ
Figure 10.28 The enhancing element in the g2b gene is orientationand position-independent. (a) Outline of the mutant plasmids.
Tonegawa and colleagues removed the X2–X3 region of the parent
plasmid containing the g2b gene (see Figure 10.27a). This deleted the
enhancer. Then they reinserted the X2–X3 fragment (with the enhancer)
in four different ways: plasmids A and B, the fragment was inserted
back into the intron in its usual location in the forward (normal)
orientation (A), or in the backward orientation (B); plasmids C and D,
the fragment was inserted into another XbaI site (X1) hundreds of base
pairs upstream of the gene in the forward orientation (C), or in the
backward orientation (D). (b) Experimental results. Tonegawa and
Expression was much more active in plasmacytoma cells.
This is also consistent with enhancer behavior because
fibroblasts do not make antibodies and therefore should
not contain enhancer-binding proteins capable of activating the enhancer of an antibody gene. Thus, the antibody
gene should not be expressed actively in such cells.
The finding that a gene is much more active in one cell
type than in another leads to an extremely important
point: All cells contain the same genes, but different cell
types differ greatly from one another: A nerve cell, for example, is much different from a liver cell, in shape and
function. What makes these cells differ so much? The proteins in the cells. And, as we have learned, the suite of
proteins in each cell type is determined by the genes that
are active in those cells. And what activates those genes?
We now see that the activators are transcription factors
that bind to enhancers. Thus, different cell types express
different activators that turn on different genes that produce different proteins. We will expand on this vital theme
in several chapters to follow.
Silencers
Enhancers are not the only DNA elements that can act at
a distance to modulate transcription. Silencers also do this,
but—as their name implies—they inhibit rather than stimulate transcription. The mating system (MAT) of yeast
provides a good example. Yeast chromosome III contains
three loci of very similar sequence: MAT, HML, and HMR.
Though MAT is expressed, the other two loci are not, and
colleagues tested all four plasmids from (a), as well as the parent, for
efficiency of expression as in Figure 10.27b. All functioned equally well.
Lane 1, untransfected J558L cells lacking the g2b gene. Lanes 2–12,
J558L cells transfected with the following plasmids: lane 2, the parent
plasmid with no deletions; lanes 3 and 4, the parent plasmid with the
X2–X3 fragment deleted; lanes 5 and 6, plasmid A; lanes 7 and 8,
plasmid B; lanes 9 and 10, plasmid C; lanes 11 and 12, plasmid D.
Lane M contained protein size markers. (Source: (a) Adapted from Gillies,
S.D., S.L. Morrison, V.T. Oi, and S. Tonegawa, A tissue-specific transcription
enhancer element is located in the major intron of a rearranged immunoglobulin
heavy chain gene. Cell 33 (July 1983) p. 721, f. 5.)
silencers located at least 1 kb away seem to be responsible
for this genetic inactivity. We know that something besides
the inactive genes themselves is at fault, because active
yeast genes can be substituted for HML or HMR and the
transplanted genes become inactive. Thus, they seem to be
responding to an external negative influence: a silencer.
How do silencers work? The available data indicate that
they cause the chromatin to coil up into a condensed, inaccessible, and therefore inactive form, thereby preventing
transcription of neighboring genes. We will examine this
process in more detail in Chapter 13.
Sometimes the same DNA element can have both enhancer and silencer activity, depending on the protein
bound to it. For example, the thyroid hormone response
element acts as a silencer when the thyroid hormone receptor binds to it without its ligand, thyroid hormone. But it
acts as an enhancer when the thyroid hormone receptor
binds along with thyroid hormone. We will revisit this concept in Chapter 12.
SUMMARY Enhancers and silencers are positionand orientation-independent DNA elements that
stimulate or depress, respectively, the transcription
of associated genes. They are also tissue-specific in
that they rely on tissue-specific DNA-binding proteins for their activities. Sometimes a DNA element
can act as either an enhancer or a silencer depending
on what is bound to it.
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Chapter 10 / Eukaryotic RNA Polymerases and Their Promoters
S U M M A RY
Eukaryotic nuclei contain three RNA polymerases that
can be separated by ion-exchange chromatography. RNA
polymerase I is found in the nucleolus; the other two
polymerases are located in the nucleoplasm. The three
nuclear RNA polymerases have different roles in
transcription. Polymerase I makes a large precursor to the
major rRNAs (5.8S, 18S, and 28S rRNAs in vertebrates).
Polymerase II synthesizes hnRNAs, which are precursors
to mRNAs. It also makes miRNA precursors and most
small nuclear RNAs (snRNAs). Polymerase III makes the
precursors to 5S rRNA, the tRNAs, and several other
small cellular and viral RNAs.
The subunit structures of all three nuclear polymerases
from several eukaryotes have been determined. All of these
structures contain many subunits, including two large
ones, with molecular masses greater than 100 kD. All
eukaryotes seem to have at least some common subunits
that are found in all three polymerases. The genes
encoding all 12 RNA polymerase II subunits in yeast have
been sequenced and subjected to mutation analysis. Three
of the subunits resemble the core subunits of bacterial
RNA polymerases in both structure and function, five are
found in all three nuclear RNA polymerases, two are not
required for activity, at least at normal temperatures, and
two fall into none of these three categories.
Subunit IIa is the primary product of the RPB1 gene in
yeast. It can be converted to IIb in vitro by proteolytic
removal of the carboxyl-terminal domain (CTD), which is
essentially a heptapeptide repeated over and over. Subunit
IIa is converted in vivo to IIo by phosphorylating two
serines within the CTD heptad. The enzyme (polymerase
IIA) with the IIa subunit is the one that binds to the
promoter; the enzyme (polymerase IIO) with the IIo
subunit is the one involved in transcript elongation.
The structure of yeast pol II D4/7 reveals a deep cleft
that can accept a DNA template. The catalytic center,
containing a Mg21 ion lies at the bottom of the cleft. A
second Mg21 ion is present in low concentration and
presumably enters the enzyme bound to each substrate
nucleotide.
The crystal structure of a transcription elongation
complex involving yeast RNA polymerase II (lacking
Rpb4/7) reveals that the clamp is indeed closed over the
RNA–DNA hybrid in the enzyme’s cleft, ensuring
processivity of transcription. In addition, three loops of the
clamp—the rudder, lid, and zipper—appear to play
important roles in, respectively: initiating dissociation of the
RNA–DNA hybrid, maintaining this dissociation, and
maintaining dissociation of the template DNA. The active
center of the enzyme lies at the end of pore 1, which appears
to be the conduit for nucleotides to enter the enzyme and
for extruded RNA to exit the enzyme during backtracking.
A bridge helix lies adjacent to the active center, and flexing
of this helix could play a role in translocation during
transcription. The toxin a-amanitin appears to interfere
with this flexing and thereby blocks translocation.
In moving through the entry pore toward the active site
of RNA polymerase II, an incoming nucleotide first
encounters the E (entry) site, where it is inverted relative
to its position in the A site, the active site where
phosphodiester bonds are formed. Two metal ions (Mg21
or Mn21) are present at the active site. One is permanently
bound to the enzyme and one enters the active site
complexed to the incoming nucleotide. The trigger loop
of Rpb1 positions the substrate for incorporation and
discriminates against improper nucleotides.
The structure of the 12-subunit RNA polymerase II
reveals that, with Rpb4/7 in place, the clamp is forced
shut. Because initiation occurs with the 12-subunit
enzyme, with its clamp shut, it appears that the promoter
DNA must melt before the template DNA strand can
descend into the enzyme’s active site. It also appears that
Rpb4/7 extends the dock region of the polymerase,
making it easier for certain general transcription factors
to bind, thereby facilitating transcription initiation.
Class II promoters may consist of a core promoter
immediately surrounding the transcription start site, and a
proximal promoter farther upstream. The core promoter
may contain up to six conserved elements: the TFIIB
recognition element (BRE), the TATA box, the initiator
(Inr), the downstream core element (DCE), the motif ten
element (MTE), and the downstream promoter element
(DPE). At least one of these elements is missing in most
promoters. Promoters for highly expressed specialized
genes tend to have TATA boxes, but promoters for
housekeeping genes tend to lack them.
Proximal promoter elements are usually found
upstream of class II core promoters. They differ from
the core promoter in that they bind to relatively genespecific transcription factors. For example, GC boxes
bind the transcription factor Sp1, while CCAAT boxes
bind CTF. The proximal promoter elements, unlike
the core promoter, can be orientation-independent, but
they are relatively position-dependent, unlike classical
enhancers.
Class I promoters are not well conserved in sequence
from one species to another, but the general architecture
of the promoter is well conserved. It consists of two
elements: a core element surrounding the transcription
start site, and an upstream promoter element (UPE) about
100 bp farther upstream. The spacing between these two
elements is important.
RNA polymerase III transcribes a set of short genes.
The classical class III genes (types I and II) have promoters
that lie wholly within the genes. The internal promoter
of the type I class III gene (the 5S rRNA gene) is split into
three regions: box A, a short intermediate element, and
box C. The internal promoters of the type II genes (e.g.,
the tRNA gene) are split into two parts: box A and box B.
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Analytical Questions
Other class III genes called type III (e.g., 7SK, and U6
RNA genes) lack internal promoters altogether and
contain promoters that strongly resemble class II
promoters in that they lie in the 59-flanking region and
contain TATA boxes. The U1 and U6 snRNA genes have
nonclassical class II and III promoters, respectively. The
U1 snRNA promoter has an essential proximal sequence
element (PSE) and a distal sequence element (DSE). The
U6 snRNA promoter has a PSE, a DSE, and a TATA box.
Enhancers and silencers are position- and orientationindependent DNA elements that stimulate or depress,
respectively, the transcription of associated genes. They
are also tissue-specific in that they rely on tissue-specific
DNA-binding proteins for their activities.
REVIEW QUESTIONS
1. Diagram the elution pattern of the eukaryotic nuclear RNA
polymerases from DEAE-Sephadex chromatography. Show
what you would expect if you assayed the same fractions in
the presence of 1 mg/mL of a-amanitin.
271
15. What role does the polymerase II trigger loop play in nucleotide selection? Illustrate with a schematic diagram of contacts
to the base, sugar, and triphosphate.
16. What role does the Rpb4/7 complex play in opening or
closing the clamp of RNA polymerase II? What evidence
supports this role?
17. The 12-subunit RNA polymerase II interacts with promoter
DNA. What implications does this have for the state of the
promoter DNA with which the polymerase must interact?
18. Draw a diagram of a composite polymerase II promoter,
showing all of the types of elements it could have.
19. What kinds of genes tend to have TATA boxes? What kinds
of genes tend not to have them?
20. What is the probable relationship between TATA boxes and
DPEs?
21. What are the two most likely effects of removing the TATA
box from a class II promoter?
22. Describe the process of linker scanning. What kind of information does it give?
23. List two common proximal promoter elements of class II
promoters. How do they differ from core promoter elements?
24. Diagram a typical class I promoter.
2. Describe and give the results of an experiment that shows that
polymerase I is located primarily in the nucleolus of the cell.
25. How were the elements of class I promoters discovered?
Present experimental results.
3. Describe and give the results of an experiment that shows
that polymerase III makes tRNA and 5S rRNA.
26. Describe and give the results of an experiment that shows
the importance of spacing between the elements of a class I
promoter.
4. How many subunits does yeast RNA polymerase II have?
Which of these are “core” subunits? How many subunits
are common to all three nuclear RNA polymerases?
5. Describe how epitope tagging can be used to purify polymerase II from yeast in one step.
6. Some preparations of polymerase II show three different
forms of the largest subunit (RPB1). Give the names of
these subunits and show their relative positions after SDSPAGE. What are the differences among these subunits?
Present evidence for these conclusions.
27. Compare and contrast (with diagrams) the classical and
nonclassical class III promoters. Give an example of each.
28. Diagram the structures of the U1 and U6 snRNA promoters.
Which RNA polymerase transcribes each? What is the effect
of moving the TATA box from one of these promoters to
the other? Why does this seem paradoxical?
29. Describe and give the results of an experiment that locates
the 59-border of the 5S rRNA gene’s promoter.
30. Explain the fact that enhancer activity is tissue-specific.
7. What is the structure of the CTD of RPB1?
8. Draw a rough diagram of the structure of yeast RNA polymerase II. Show where the DNA lies, and provide another
piece of evidence that supports this location for DNA. Also,
show the location of the active site.
9. How many Mg21 ions are proposed to participate in catalysis at the active center of RNA polymerases? Why is one of
these metal ions difficult to see in the crystal structure of
yeast RNA polymerase II?
10. Cite evidence to support pore 1 as the likely exit point for
RNA extrusion during polymerase II backtracking.
11. What is meant by the term “processive transcription?” What
part of the polymerase II structure ensures processivity?
12. What is the probable function of the rudder of
polymerase II?
13. What is the probable function of the bridge helix? What is
the relationship of a-amanitin to this function?
14. What are the E site and A site of RNA polymerase II? What
roles are they thought to play in nucleotide selection?
A N A LY T I C A L Q U E S T I O N S
1. Transcription of a class II gene starts at a guanosine 25 bp
downstream of the last base of the TATA box. You delete
20 bp of DNA between this guanosine and the TATA box and
transfect cells with this mutated DNA. Will transcription
still start at the same guanosine? If not, where? How would
you locate the transcription start site?
2. You suspect that a repeated sequence just upstream of a
gene is acting as an enhancer. Describe and predict the results
of an experiment you would run to test your hypothesis.
Be sure your experiment shows that the sequence acts as an
enhancer and not as a promoter element.
3. You are investigating a new class II promoter, but you can
find no familiar sequences. Design an experiment to locate
the promoter sequences, and show sample results.
4. Describe a primer extension assay you could use to define
the 39-end of the 5S rRNA promoter.
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Chapter 10 / Eukaryotic RNA Polymerases and Their Promoters
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