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28 64 Elongation
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Chapter 6 / The Mechanism of Transcription in Bacteria
Richard Gourse, Richard Ebright, and their colleagues
used limited proteolysis analysis to show that the a-subunit
N-terminal and C-terminal domains (the a-NTD and a-CTD,
respectively) fold independently to form two domains that are
tethered together by a flexible linker. A protein domain is a
part of a protein that folds independently to form a defined
structure. Because of their folding, domains tend to resist proteolysis, so limited digestion with a proteolytic enzyme will
attack unstructured elements between domains and leave the
domains themselves alone. When Gourse and Ebright and collaborators performed limited proteolysis on the E. coli RNA
polymerase a-subunit, they released a polypeptide of about
28 kD, and three polypeptides of about 8 kD. The sequences
of the ends of these products showed that the 28-kD polypeptide contained amino acids 8–241, whereas the three small
polypeptides contained amino acids 242–329, 245–329, and
249–329. This suggested that the a-subunit folds into two
domains: a large N-terminal domain encompassing (approximately) amino acids 8–241, and a small C-terminal domain
including (approximately) amino acids 249–329.
Furthermore, these two domains appear to be joined by
an unstructured linker that can be cleaved in at least three
places by the protease used in this experiment (Glu-C). This
linker seems at first glance to include amino acids 242–248.
Because Glu-C requires three unstructured amino acids on
either side of the bond that is cleaved, however, the linker is
longer than it appears at first. In fact, it must be at least 13
amino acids long (residues 239–251).
These experiments suggest a model such as the one presented in Figure 6.27. RNA polymerase binds to a core
promoter via its s-factor, with no help from the C-terminal
domains of its a-subunits, but it binds to a promoter with
an UP element using s plus the a-subunit C-terminal domains. This allows very strong interaction between polymerase and promoter and therefore produces a high level
of transcription.
(a)
αNTD
αCTD
β
σ
β′
−10
−35
(b)
αNTD
αCTD
UP
β
σ
−35
β′
−10
Figure 6.27 Model for the function of the C-terminal domain (CTD)
of the polymerase a-subunit. (a) In a core promoter, the a-CTDs are
not used, but (b) in a promoter with an UP element, the a-CTDs
contact the UP element. Notice that two a-subunits are depicted: one
behind the other.
SUMMARY The RNA polymerase a-subunit has an
independently folded C-terminal domain that can
recognize and bind to a promoter’s UP element. This
allows very tight binding between polymerase and
promoter.
6.4
Elongation
After initiation of transcription is accomplished, the core
continues to elongate the RNA, adding one nucleotide after
another to the growing RNA chain. In this section we will
explore this elongation process.
Core Polymerase Functions in Elongation
So far we have been focusing on the role of s because of the
importance of this factor in determining the specificity of
initiation. However, the core polymerase contains the RNA
synthesizing machinery, so the core is the central player in
elongation. In this section we will see evidence that the
b- and b9-subunits are involved in phosphodiester bond
formation, that these subunits also participate in DNA
binding, and that the a-subunit has several activities, including assembly of the core polymerase.
The Role of b in Phosphodiester Bond Formation Walter
Zillig was the first to investigate the individual core subunits, in 1970. He began by separating the E. coli core
polymerase into its three component polypeptides and then
combining them again to reconstitute an active enzyme.
The separation procedure worked as follows: Alfred Heil
and Zillig electrophoresed the core enzyme on cellulose
acetate in the presence of urea. Like SDS, urea is a denaturing agent that can separate the individual polypeptides in a
complex protein. Unlike SDS, however, urea is a mild denaturant that is relatively easy to remove. Thus, it is easier to
renature a urea-denatured polypeptide than an SDSdenatured one. After electrophoresis was complete, Heil
and Zillig cut out the strips of cellulose acetate containing
the polymerase subunits and spun them in a centrifuge to
drive the buffer, along with the protein, out of the cellulose
acetate. This gave them all three separated polypeptides,
which they electrophoresed individually to demonstrate
their purity (Figure 6.28).
Once they had separated the subunits, they recombined
them to form active enzyme, a process that worked best
in the presence of s. Using this separation–reconstitution
system, Heil and Zillig could mix and match the components from different sources to answer questions about
their functions. For example, recall that the core polymerase
determines sensitivity or resistance to the antibiotic rifampicin, and that rifampicin blocks transcription initiation.
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6.4 Elongation
1
2
3
resistance or sensitivity. At first this seems paradoxical. How
can the same core subunit be involved in both initiation
and elongation? The answer, which we will discuss in detail
later in this chapter, is that rifampicin actually blocks early
elongation, preventing the RNA from growing more than
2–3 nucleotides long. Thus, strictly speaking, it blocks initiation, because initiation is not complete until the RNA is
up to 10 nucleotides long, but its effect is really on the
elongation that is part of initiation.
In 1987, M. A. Grachev and colleagues provided more
evidence for the notion that b plays a role in elongation,
using a technique called affinity labeling. The idea behind
this technique is to label an enzyme with a derivative of a
normal substrate that can be cross-linked to protein. In
this way, one can use the affinity reagent to seek out and
then tag the active site of the enzyme. Finally, one can dissociate the enzyme to see which subunit the tag is attached
to. Grachev and coworkers used 14 different affinity
reagents, all ATP or GTP analogs. One of these, which was
the first in the series, and therefore called I, has the structure shown in Figure 6.30a. When it was added to RNA
polymerase, it went to the active site, as an ATP that is
initiating transcription would normally do, and then
formed a covalent bond with an amino group at the active
site according to the reaction in Figure 6.30b.
In principle, these investigators could have labeled the
affinity reagent itself and proceeded from there. However,
they recognized a pitfall in that simple strategy: The affinity
reagent could bind to other amino groups on the enzyme
surface in addition to the one(s) in the active site. To circumvent this problem, they used an unlabeled affinity
reagent, followed by a radioactive nucleotide ([a-32P]UTP or
CTP) that would form a phosphodiester bond with the
affinity reagent in the active site and therefore label that
site and no others on the enzyme. Finally, they dissociated
the labeled enzyme and subjected the subunits to SDS-PAGE.
4
β′
β
α
Figure 6.28 Purification of the individual subunits of E. coli RNA
polymerase. Heil and Zillig subjected the E. coli core polymerase to
urea gel electrophoresis on cellulose acetate, then collected the
separated polypeptides. Lane 1, core polymerase after electrophoresis;
lane 2, purified a; lane 3, purified b; lane 4, purified b9. (Source: Heil, A.
and Zillig, W. Reconstitution of bacterial DNA-dependent RNA-polymerase from
isolated subunits as a tool for the elucidation of the role of the subunits in
transcription. FEBS Letters 11 (Dec 1970) p. 166, f. 1.)
Separation and reconstitution of the core allowed Heil and
Zillig to ask which core subunit confers this antibiotic
sensitivity or resistance. When they recombined the a-, b9-,
and s-subunits from a rifampicin-sensitive bacterium with
the b-subunit from a rifampicin-resistant bacterium, the
resulting polymerase was antibiotic-resistant (Figure 6.29).
Conversely, when the b-subunit came from an antibioticsensitive bacterium, the reconstituted enzyme was antibioticsensitive, regardless of the origin of the other subunits.
Thus, the b-subunit is obviously the determinant of rifampicin sensitivity or resistance.
Another antibiotic, known as streptolydigin, blocks
RNA chain elongation. By the same separation and reconstitution strategy used for rifampicin, Heil and Zillig
showed that the b-subunit also governed streptolydigin
β′
α
α
β
σ
Separate
α
+
β′
+
Rifampicin-sensitive
145
σ
Reconstitute
β
α
β′
α
σ
Rifampicin-resistant
β′
α
β
α
σ
Separate
β
Rifampicin-resistant
Figure 6.29 Separation and reconstitution of RNA polymerase
to locate the determinant of antibiotic resistance. Start with RNA
polymerases from rifampicin-sensitive and -resistant E. coli cells,
separate them into their component polypeptides, and recombine
them in various combinations to reconstitute the active enzyme. In this
case, the a-, b9-, and s-subunits came from the rifampicin-sensitive
polymerase (blue), and the b-subunit came from the antibiotic-resistant
enzyme (red). The reconstituted polymerase is rifampicin-resistant,
which shows that the b-subunit determines sensitivity or resistance to
this antibiotic.
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Chapter 6 / The Mechanism of Transcription in Bacteria
(a)
SUMMARY The core subunit b lies near the active
O
–O
P
O
P
O
–O
P
site of the RNA polymerase where phosphodiester
bonds are formed. The s-factor may also be near
the nucleotide-binding site, at least during the initiation phase.
O
OCH 2
O
O
A
O
OH OH
Structure of the Elongation Complex
Reagent I
Studies in the mid-1990s had suggested that the b and b9
subunits are involved in DNA binding. In this section, we
will see how well these predictions have been borne out by
structural studies. We will also consider the topology of
elongation: How does the polymerase deal with the problems of unwinding and rewinding its template, and of moving along its twisted (helical) template without twisting its
RNA product around the template?
(b)
I
+
Polymerase
NH2
Polymerase
O
O
O
H
N P O P O P O
O–
O–
O–
OCH 2
A
O
OH OH
32
P–UTP
Polymerase
O
O
H
N P O P
O–
O–
O
O P O
OCH 2
O–
O
A
O
O
OH
32P
O–
OCH 2
O
U
OH OH
Figure 6.30 Affinity labeling RNA polymerase at its active site.
(a) Structure of one of the affinity reagents (I), an ATP analog. (b) The
affinity-labeling reactions. First, add reagent I to RNA polymerase.
The reagent binds covalently to amino groups at the active site (and
perhaps elsewhere). Next, add radioactive UTP, which forms a
phosphodiester bond (blue) with the enzyme-bound reagent I. This
reaction should occur only at the active site, so only that site
becomes radioactively labeled.
The results are presented in Figure 6.31. Obviously, the
b-subunit is the only core subunit labeled by any of the affinity reagents, suggesting that this subunit is at or very
near the site where phosphodiester bond formation occurs.
In some cases, we also see some labeling of s, suggesting
that it too may lie near the catalytic center.
The RNA–DNA Hybrid Up to this point we have been
assuming that the RNA product forms an RNA–DNA
hybrid with the DNA template strand for a few bases
before peeling off and exiting from the polymerase. But
the length of this hybrid has been controversial, with
estimates ranging from 3–12 bp, and some investigators
even doubted whether it existed. But Nudler and Goldfarb and their colleagues applied a transcript walking
technique, together with RNA–DNA cross-linking, to
prove that an RNA–DNA hybrid really does occur
within the elongation complex, and that this hybrid is
8–9 bp long.
The transcript walking technique works like this:
Nudler and colleagues used gene cloning techniques
described in Chapter 4 to engineer an RNA polymerase
with six extra histidines at the C-terminus of the b–subunit.
This string of histidines, because of its affinity for divalent
metals such as nickel, allowed them to tether the polymerase to a nickel resin so they could change substrates
rapidly by washing the resin, with the polymerase stably
attached, and then adding fresh reagents. Accordingly, by
adding a subset of nucleotides (e.g., ATP, CTP, and GTP,
but no UTP), they could “walk” the polymerase to a particular position on the template (where the first UTP
is required, in the present case). Then they could wash
away the first set of nucleotides and add a second subset
to walk the polymerase to a defined position further
downstream.
These workers incorporated a UMP derivative (U•) at
either position 21 or 45 with respect to the 59-end of a
32
P-labeled nascent RNA. U• is normally unreactive, but
in the presence of NaBH4 it becomes capable of crosslinking to a base-paired base, as shown in Figure 6.32a.
Actually, U• can reach to a purine adjacent to the basepaired A in the DNA strand, but this experiment was
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6.4 Elongation
1
2
3
4
5
6
7
8
9
10
11 12 13 14
15
147
16 17
β′
β
β′
β
σ
σ
α
α
Figure 6.31 The b-subunit is at or near the active site where
phosphodiester bonds are formed. Grachev and colleagues labeled
the active site of E. coli RNA polymerase as described in Figure 6.30,
then separated the polymerase subunits by electrophoresis to identify
the subunits that compose the active site. Each lane represents
labeling with a different nucleotide-affinity reagent plus radioactive UTP,
except lanes 5 and 6, which resulted from using the same affinity
reagent, but either radioactive UTP (lane 5) or CTP (lane 6). The
autoradiograph of the separated subunits demonstrates labeling of the
b-subunit with most of the reagents. In a few cases, s was also faintly
labeled. Thus, the b-subunit appears to be at or near the
phosphodiester bond-forming active site. (Source: Grachev et al., Studies on
(a)
(b)
HOCH2
O
C
CH2
+
N
CH2
R
NH
H
O
H2C
CH CH
N
H
the functional topography of Escherichia coli RNA polymerase. European Journal of
Biochemistry 163 (16 Dec 1987) p. 117, f. 2.)
1
2
3
4
5
6
7
8
9 10 11 12 13 14 15 16 17 18
Protein
DNA
N
H
U position –
N
N
N
N
– –2 –3 –5 –6 –7 –10 –14 –24 – –3 –6 –7 –8 –10 –13 –18
To DNA
template
strand
RNA
U
N
O
A
To RNA
RNA 3' end 20 22 22* 23* 25* 26* 27* 30* 34* 44* 44 47* 50* 51* 52* 54* 57* 62*
Figure 6.32 RNA–DNA and RNA–protein cross-linking in elongation complexes. (a) Structure of the cross-linking reagent U• base-paired with
an A in the DNA template strand. The reagent is in position to form a covalent bond with the DNA as shown by the arrow. (b) Results of cross-linking.
Nudler, Goldfarb, and colleagues incorporated U• at position 21 or 45 of a [32P]nascent RNA in an elongation complex. Then they walked the U• to
various positions between 22 and 224 with respect to the 39-end (position 21) of the nascent RNA. Then they cross-linked the RNA to the DNA
template (or the protein in the RNA polymerase). They then electrophoresed the DNA and protein in one gel (top) and the free RNA transcripts in
another (bottom) and autoradiographed the gels. Lanes 1, 2, and 11 are negative controls in which the RNA contained no U•. Lanes 3210 contained
products from reactions in which the U• was in position 21; lanes 12–18 contained products from reactions in which the U• was in position 45 of
the nascent RNA. Asterisks at bottom denote the presence of U• in the RNA. Cross-linking to DNA was prevalent only when U• was between
positions 22 and 28. (Sources: (a) Reprinted from Cell 89, Nudler, E. et al. The RNA-DNA hybrid maintains the register of transcription by preventing backtracking of RNA
polymerase fig.1, p. 34 © 1997 from Elsevier (b) Nudler, E. et al. The RNA–DNA hybrid maintains the register of transcription by preventing backtracking of RNA polymerase.
Cell 89 (1997) f. 1, p. 34. Reprinted by permission of Elsevier Science.)
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Chapter 6 / The Mechanism of Transcription in Bacteria
designed to prevent that from happening. So cross-linking
could occur only to an A in the DNA template strand
that was base-paired to the U• base in the RNA product.
If no base-pairing occurred, no cross-linking would be
possible.
Nudler, Goldfarb, and their colleagues walked the U•
base in the transcript to various positions with respect to
the 39-end of the RNA, beginning with position 22 (the
nucleotide next to the 39-end, which is numbered 21) and
extending to position 244. Then they tried to cross-link
the RNA to the DNA template strand. Finally, they electrophoresed both the DNA and protein in one gel, and just the
RNA in another. Note that the RNA will always be labeled,
but the DNA or protein will be labeled only if the RNA has
been cross-linked to them.
Figure 6.32b shows the results. The DNA was
strongly labeled if the U• base was in position 22
through position 28, but only weakly labeled when the
U• base was in position 210 and beyond. Thus, the U•
base was base-paired to its A partner in the DNA template strand only when it was in position 22 through
28, but base-pairing was much decreased when the reactive base was in position 210. So the RNA–DNA hybrid
extends from position 21 to position 28, or perhaps
29, but no farther. (The nucleotide at the very 39-end of
the RNA, at position 21, must be base-paired to the
template to be incorporated correctly.) This conclusion
was reinforced by the protein labeling results. Protein in
the RNA polymerase became more strongly labeled
when the U• was not within the hybrid region (positions
21 through 28). This presumably reflects the fact that
the reactive group was more accessible to the protein
when it was not base-paired to the DNA template. More
recent work on the T7 RNA polymerase has indicated a
hybrid that is 8 bp long.
SUMMARY The RNA–DNA hybrid within the E. coli
elongation complex extends from position 21 to
position 28 or 29 with respect to the 39-end of the
nascent RNA. The T7 hybrid appears to be 8 bp long.
Structure of the Core Polymerase To get the clearest picture of the structure of the elongation complex, we need to
know the structure of the core polymerase. X-ray crystallography would give the best resolution, but it requires
three-dimensional crystals and, so far, no one has succeeded
in preparing three-dimensional crystals of the E. coli
polymerase. However, in 1999 Seth Darst and colleagues
crystallized the core polymerase from another bacterium,
Thermus aquaticus, and obtained a crystal structure to a
resolution of 3.3 Å. This structure is very similar in overall
shape to the lower-resolution structure of the E. coli
core polymerase obtained by electron microscopy of
two-dimensional crystals, so the detailed structures are
probably also similar. In other words, the crystal structure
of the T. aquaticus polymerase is our best window right
now on the structure of a bacterial polymerase. As we look
at this and other crystal structures throughout this book,
we need to remember a principle we will discuss more fully
in Chapters 9 and 10: Proteins do not have just one static
structure. Instead, they are dynamic molecules that can assume a wide range of conformations. The one we trap in a
crystal may not be the one (or more than one) that the active form of the protein assumes in vivo.
Figure 6.33 depicts the overall shape of the enzyme in
three different orientations. We notice first of all that it
resembles an open crab claw. The four subunits (b, b9,
and two a) are shown in different colors so we can distinguish them. This coloring reveals that half of the claw
is composed primarily of the b-subunit, and the other
half is composed primarily of the b9-subunit. The two
a- subunits lie at the “hinge” of the claw, with one
of them (aI, yellow) associated with the b-subunit, and
the other (aII, green) associated with the b9-subunit. The
small v-subunit is at the bottom, wrapped around the
C-terminus of b9.
Figure 6.34 shows the catalytic center of the core polymerase. We see that the enzyme contains a channel, about
27 Å wide, between the two parts of the claw, and the template DNA presumably lies in this channel. The catalytic
center of the enzyme is marked by the Mg21 ion, represented here by a pink sphere. Three pieces of evidence
place the Mg21 at the catalytic center. First, an invariant
string of amino acids (NADFDGD) occurs in the b9-subunit
from all bacteria examined so far, and it contains three
aspartate residues (D) suspected of chelating a Mg21
ion. Second, mutations in any of these Asp residues are
lethal. They create an enzyme that can form an openpromoter complex at a promoter, but is devoid of catalytic
activity. Thus, these Asp residues are essential for catalytic
activity, but not for tight binding to DNA. Finally, as
Figure 6.34 demonstrates, the crystal structure of the
T. aquaticus core polymerase shows that the side chains of
the three Asp residues (red) are indeed coordinated to a
Mg21 ion. Thus, the three Asp residues and a Mg21 ion
are at the catalytic center of the enzyme.
Figure 6.34 also identifies a rifampicin-binding site
in the part of the b-subunit that forms the ceiling of
the channel through the enzyme. The amino acids
whose alterations cause rifampicin resistance are
tagged with purple dots. Clearly, these amino acids are
tightly clustered in the three-dimensional structure,
presumably at the site of rifampicin binding. We also
know that rifampicin allows RNA synthesis to begin,
but blocks elongation of the RNA chain beyond just a
few nucleotides. On the other hand, the antibiotic has
no effect on elongation once promoter clearance has
occurred.
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6.4 Elongation
149
Figure 6.33 Crystal structure of the Thermus aquaticus RNA polymerase core enzyme. Three different stereo views are shown, differing by
90-degree rotations. The subunits and metal ions in the enzyme are color-coded as indicated at the bottom. The metal ions are depicted as small
colored spheres. The larger red dots denote unstructured regions of the b- and b9-subunits that are missing from these diagrams.
(Source: Zhang, G. et al., Crystal structure of Thermus aquaticus core RNA polymerase at 3.3 Å resolution. Cell 98 (1999) 811–24. Reprinted by permission of Elsevier Science.)
How can we interpret the location of the rifampicinbinding site in terms of the antibiotic’s activity? One
hypothesis is that rifampicin bound in the channel blocks
the exit through which the growing RNA should pass, and
thus prevents growth of a short RNA. Once an RNA reaches a
certain length, it might block access to the rifampicin-binding
site, or at least prevent effective binding of the antibiotic.
Darst and colleagues validated this hypothesis by determining the crystal structure of the T. aquaticus polymerase
core complexed with rifampicin. The antibiotic lies in the
predicted site in such a way that it would block the exit of
the elongating transcript when the RNA reaches a length
of 2 or 3 nt.
SUMMARY X-ray crystallography on the Thermus
aquaticus RNA polymerase core has revealed an enzyme shaped like a crab claw designed to grasp DNA.
A channel through the enzyme includes the catalytic
center (a Mg21 ion coordinated by three Asp residues), and the rifampicin-binding site.
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Figure 6.34 Stereo view of the catalytic center of the core polymerase. The Mg21 ion is shown as a pink sphere, coordinated by three
aspartate side chains (red) in this stereo image. The amino acids involved in rifampicin resistance are denoted by purple spheres at the top of the
channel, surrounding the presumed rifampicin-binding site, or Rif pocket, labeled Rif r. The colors of the polymerase subunits are as in Figure 6.33
(b9, pink; b, turquoise; a’s yellow and green). Note that the two panels of this figure are the two halves of the stereo image. (Source: Zhang G. et al.,
“Crystal structure of Thermus aquaticus core RNA polymerase at 3.3 Å resolution.” Cell 98 (1999) 811–24. Reprinted by permission of Elsevier and Green Science.)
Nontemplate strand
–40
–30
–20
–10
5′
3′
3′
5′
–35 box
–10 box
Template strand
Extended
–10 box
Figure 6.35 Structure of the DNA used to form the RF complex.
The 210 and 235 boxes are shaded yellow, and an extended 210
element is shaded red. Bases 211 through 27 are in single-stranded
form, as they would be in an open promoter complex.
Structure of the Holoenzyme–DNA Complex To generate a homogeneous holoenzyme–DNA complex, Darst
and colleagues bound the T. aquaticus holoenzyme to the
“fork-junction” DNA pictured in Figure 6.35. This DNA
is mostly double-stranded, including the 235 box, but has
a single-stranded projection on the nontemplate strand in
the 210 box region, beginning at position 211. This
simulates the character of the promoter in the open promoter complex, and locks the complex into a form (RF,
where F stands for “fork junction”) resembling RPo.
Figure 6.36a shows an overall view of the holoenzyme–
promoter complex. The first thing to notice is that the DNA
stretches across the top of the polymerase in this view—
where the s-subunit is located. In fact, all of the specific
DNA–protein interactions involve s, not the core. Considering the importance of s in initiation, that is not surprising.
Looking more closely (Figure 6.36b) we can see that the
structure corroborates several features already inferred
from biochemical and genetic experiments. First of all, as
we saw earlier in this chapter, s region 2.4 is implicated in
recognizing the 210 box of the promoter. In particular,
mutations in Gln 437 and Thr 440 of E. coli s 70 can suppress
mutations in position 212 of the promoter, suggesting an
interaction between these two amino acids and the base at
position 212 (recall Figure 6.22). Gln 437 and Thr 440 in
E. coli s70 correspond to Gln 260 and Asn 263 of
T. aquaticus sA, so we would expect these two amino acids
to be close to the base at position 212 in the promoter.
Figure 6.36b bears out part of this prediction. Gln 260
(Q260, green) is indeed close enough to contact base 212.
Asn 263 (N263, also colored green) is too far away to make
contact in this structure, but a minor movement, which
could easily occur in vivo, would bring it close enough.
Three highly conserved aromatic residues in E. coli s70
(corresponding to Phe 248 (F248), Tyr 253 (Y253), and
Trp 256 (W256) of T. aquaticus sA) have been implicated
in promoter melting. These amino acids presumably bind
the nontemplate strand in the 210 box in the open promoter complex. These amino acids (colored yellow-green
in Figure 6.36b) are indeed in position to interact with the
single-stranded nontemplate strand in the RF complex. In
fact, Trp 256 is neatly positioned to stack with base pair
12, which is the last base pair before the melted region of
the 210 box. In this way, Trp 256 would substitute for a
base pair in position 211 and help melt that base pair.
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6.4 Elongation
Upstream
–35
element
–30
–40
151
Downstream
Extended
–10 element
–20
5′nt
–10
element
3′nt
β′
β
(a)
(b)
Figure 6.36 Structure of the RF complex. (a) The whole complex. The various subunits are color coded as follows: b, turquoise; b’, brown; a,
gray; regions of s (s22s4), tan and orange (s1 is not included in this crystal structure). The DNA is shown as a twisted ladder. The surface of s is
rendered partially transparent to reveal the path of the a-carbon backbone. (b) Contacts between the holoenzyme and downstream DNA. The s2
and s3 domains are colored as in (a), except for residues that have been implicated by genetic studies in downstream promoter binding. These are:
extended 210 box recognition, red; 210 box recognition, green; 210 box melting and nontemplate strand binding, yellow-green; and invariant basic
residues implicated in DNA binding, blue. The 210 box DNA is yellow and the extended 210 box DNA is red. The 3’-end of the nontemplate strand
is denoted 39nt. Specific amino acid side chains that are important in DNA binding are labeled. The box in the small structure at lower right shows
the position of the magnified structure within the RF complex. (Source: Murakami et al., Science 296: (a), p. 1287; (b), p. 1288. Copyright 2002 by the AAAS.)
Two invariant basic residues in s regions 2.2 and 2.3
(Arg 237 [R237] and Lys 241 [K241]) are known to participate in DNA binding. Figure 6.36b shows why: These two
residues (colored blue in the figure) are well positioned to
bind to the acidic DNA backbone by electrostatic interaction.
These interactions are probably not sequence-specific.
Previous studies implicated region 3 of s in DNA binding, in particular binding to the extended (upstream) 210
box. Specifically, Glu 281 (E281) was found to be important
in recognizing the extended 210 box, while His 278 (H278)
was implicated in more general DNA-binding in this region.
The structure in Figure 6.36b is consistent with those findings: Both Glu 281 and His 278 (red shading on s region 3)
are exposed on an a-helix, and face the major groove of the
extended 210 box (red DNA). Glu 281 is probably close
enough to contact a thymine at position 213, and His 278 is
close enough to the extended 210 box that it could interact
nonspecifically with the phosphodiester bond linking the
nontemplate strand residues 217 and 218.
We saw earlier in this chapter that specific residues in s
region 4.2 are instrumental in binding to the 235 box of
the promoter. But, surprisingly, the RF structure does not
confirm these findings. In particular, the 235 box seems
about 6 Å out of position relative to s4.2, and the DNA is
straight instead of bending to make the necessary
interactions. Because the evidence for these 235 box–s4.2
interactions is so strong, Darst and colleagues needed to
explain why their crystal structure does not allow them.
They concluded that the 235 box DNA in the RF structure
is pushed out of its normal position relative to s4.2 by crystal packing forces—a reminder that the shape a molecule or
a complex assumes in a crystal is not necessarily the same as
its shape in vivo, and indeed that proteins are dynamic molecules that can change shape as they do their jobs.
The studies of Darst and colleagues, and others, have
revealed only one Mg21 ion at the active site. But all DNA
and RNA polymerases are thought to use a mechanism
that requires two Mg21 ions. In accord with this mechanism, Dmitry Vassylyev and colleagues have determined
the crystal structure of the T. thermophilus polymerase at
2.6 Å resolution. Their asymmetric crystals contained two
polymerases, one with one Mg21 ion, and one with two.
The latter is probably the form of the enzyme that takes
part in RNA synthesis. The two Mg21 ions are held by the
same three aspartate side chains that hold the single Mg21
ion, in a network involving several nearby water molecules.
SUMMARY The crystal structure of a Thermus
aquaticus holoenzyme–DNA complex mimicking
an open promoter complex reveals several things.
First, the DNA is bound mainly to the s-subunit,
which makes all the important interactions with the
promoter DNA. Second, the predicted interactions
between amino acids in region 2.4 of s and the
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210 box of the promoter are really possible. Third,
three highly conserved aromatic amino acids are
predicted to participate in promoter melting, and
they really are in a position to do so. Fourth, two
invariant basic amino acids in s are predicted to participate in DNA binding and they are in a position to
do so. A higher resolution crystal structure reveals a
form of the polymerase that has two Mg21 ions, in
accord with the probable mechanism of catalysis.
Structure of the Elongation Complex In 2007, Dmitry
Vassylyev and colleagues presented the x-ray crystal
structure of the Thermus thermophilus RNA polymerase
elongation complex at 2.5Å resolution. This complex
contained 14 bp of downstream double-stranded DNA
that had yet to be melted by the polymerase, 9 bp of
RNA–DNA hybrid, and 7 nt of RNA product in the
RNA exit channel. Several important observations came
from this work.
First, a valine residue in the b9 subunit inserts into the
minor groove of the downstream DNA. This could have
two important consequences: It could prevent the DNA
from slipping backward or forward in the enzyme; and it
could induce the screw-like motion of the DNA through
the enzyme, which we will examine later in this chapter.
(Consider a screw being driven through a threaded hole in
a piece of metal. The metal threads, because of their position between the threads of the screw, require the screw to
turn in order to penetrate or withdraw.) There are analogous residues in the single-subunit phage T7 RNA polymerase (Chapter 8), and in the multi-subunit yeast enzyme
(Chapter 10) that probably play the same role as the valine
residue in the T. thermophilus b9 subunit.
Second, as Figure 6.37a shows, the downstream DNA
is double-stranded up to and including the 12 base pair,
where 11 is the position at which the new nucleotide is
added. This means that only one base pair (at position 11)
is melted and available for base-pairing with an incoming
nucleotide, so only one nucleotide at a time can bind specifically to the complex. Figure 6.37a also demonstrates
that one amino acid in the b subunit is situated in a key
position right at the site where nucleotides are added to the
growing RNA chain. This is arginine 422 of the b fork
2 loop. It makes a hydrogen bond with the phosphate of
the 11 template nucleotide, and van der Waals interactions
with both bases of the 12 base pair. In the T7 polymerase
elongation complex, phenylalanine 644 is in a similar position (Figure 6.37b). The proximity of these amino acids to
the active site, and their interactions with key nucleotides
there, suggests that they play a role in molding the active
site for accurate substrate recognition. If this is so, then
mutations in these amino acids should decrease the accuracy of transcription. Indeed, changing phenylalanine 644
Figure 6.37 Strand separation in the DNA template and in the
RNA–DNA hybrid. (a) Downstream DNA strand separation in the T.
thermophilus polymerase. Note the interactions between R422 (green)
and the template nucleotide phosphate and the 12 base pair. In all
panels, polar interactions are in dark blue, and van der Waals
interactions are in blue-green dashed lines. (b) Downstream DNA
strand separation in the T7 enzyme. Note the interactions between
F644 (green) and the template nucleotide phosphate and the 12 base
pair. (c) RNA–DNA hybrid strand separation in the T. thermophilus
enzyme. Note the stacking of three amino acids in the b9 lid (blue) and
the 29 base pair, and the interaction of the first displaced RNA base
(210, light green) with the pocket in the b switch 3 loop (orange).
(d) Detail of interactions between the first displaced RNA base (210)
and five amino acids in the b switch 3 loop (orange). Source: Reprinted
by permission from Macmillan Publishers Ltd: Nature, 448, 157–162, 20 June
2007. Vassylyev et al, Structural basis for transcription elongation by bacterial RNA
polymerase. © 2007.
(or glycine 645) of the T7 polymerase to alanine does decrease fidelity. At the time this work appeared, the effect of
mutations in arginine 422 of the bacterial enzyme had not
been checked.
Third, in agreement with previous biochemical work,
the enzyme can accommodate nine base pairs of RNA–
DNA hybrid. Furthermore, at the end of this hybrid, a series of amino acids of the b9 lid (valine 530, arginine 534,
and alanine 536) stack on base pair 29, stabilizing it, and
limiting any further base-pairing (Figure 6.37c). These interactions therefore appear to play a role in strand separation at the end of the RNA–DNA hybrid. A variety of
experiments have shown the hybrid to vary between 8–10 bp
in length, and the b9 lid appears to be flexible enough to
handle that kind of variability. But other forces are at work
in limiting the length of the hybrid. One is the tendency of
the two DNA strands to reanneal. Another is the trapping
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6.4 Elongation
of the first displaced RNA base (210) in a hydrophobic
pocket of a b loop known as switch 3 (Figure 6.37c). Five
amino acids in this pocket make van der Waals interactions
with the displaced RNA base (Figure 6.37d), stabilizing the
displacement.
Fourth, the RNA product in the exit channel is twisted
into the shape it would assume as one-half of an A-form
double-stranded RNA. Thus, it is ready to form a hairpin
that will cause pausing, or even termination of transcription (see later in this chapter and Chapter 8). Because RNA
in hairpin form was not used in this structural study, we
cannot see exactly how a hairpin would fit into the exit
channel. However, Vassylyev and colleagues modeled the fit
of an RNA hairpin in the exit channel, and showed that
such a fit can be accomplished with only minor alterations
of the protein structure. Indeed, the RNA hairpin could fit
with the core enzyme in much the same way as the s-factor
fits with the core in the initiation complex.
In a separate study, Vassylyev and colleagues examined
the structure of the elongation complex including an unhydrolyzable substrate analog, adenosine-59-[(a, b)-methyleno]triphosphate (AMPcPP), which has a methylene (CH2)
group instead of an oxygen between the a- and b- phosphates of ATP. Since this is the bond that is normally broken when the substrate is added to the growing RNA chain,
the substrate analog binds to the catalytic site and remains
there unaltered. These investigators also looked at the elongation complex structure with AMPcPP and with and without the elongation inhibitor streptolydigin. This comparison
yielded interesting information about how the substrate
associates with the enzyme in a two-step process.
In the absence of streptolydigin, the so-called trigger
loop (residues 1221–1266 of the b9 subunit) is fully
folded into two a-helices with a short loop in between.
(Figure 6.38a). This brings the substrate into the active
site in a productive way, with two metal ions (Mg21, in
this case) close enough together to collaborate in forming the phosphodiester bond that will incorporate the
new substrate into the growing RNA chain. Studies of
many RNA and DNA polymerases (see Chapter 10) have
shown that two metal ions participate in phosphodiester
bond formation. One of these is permanently held in the
active site, and the other shuttles in, bound to the b- and
(g-phosphates of the NTP substrate. Once the substrate
is added to the growing RNA, the second metal ion
leaves, bound to the by-product, inorganic pyrophosphate (which comes from the b- and (g-phosphates of the
substrate).
In the presence of streptolydigin, by contrast, the antibiotic forces a change in the trigger loop conformation:
The two a-helices unwind somewhat to form a larger loop
in between. This in turn forces a change in the way the
substrate binds to the active site: The base and sugar of the
substrate bind in much the same way, but the triphosphate
part extends a bit farther away from the active site, taking
(a) Pre-insertion
(+ streptolydigin)
A
P
153
(b) Insertion
(– streptolydigin)
Stl
P
A
P
B P
B P
P
Trigger loop
Trigger
helices
Figure 6.38 A two-step model for nucleotide insertion during RNA
synthesis. (a) Pre-insertion state. This is presumably a natural first
step in vivo, but it is stabilized by the antibiotic streptolydigin in vitro.
Here, streptolydigin (yellow) is forcing the trigger loop out of its
normal position close to the active site, which in turn allows the
incoming nucleotide (orange with purple triphosphate) to extend its
triphosphate moiety away from the active site (exaggerated in this
illustration). Because the second metal (metal B) essential for catalysis
is complexed to the b- and g-phosphates of the incoming nucleotide,
this places metal B too far away from metal A to participate in
catalysis. (b) Insertion state. No streptolydigin is present, so the trigger
loop can fold into trigger helices that lie closer to the active site,
allowing the triphosphates of the incoming nucleotide, and their
complexed metal B, to approach closer to metal A at the active site.
This arrangement allows the two metal ions to collaborate in
nucleotide insertion into the growing RNA chain.
with it one of the metal ions required for catalysis (Figure 6.38b). This makes catalysis impossible and explains
how streptolydigin blocks transcription elongation.
Vassylyev and colleagues concluded that the two states
of the elongation complex revealed by streptolydigin correspond to two natural states: a preinsertion state (seen in
the presence of the antibiotic) and an insertion state (seen
in the absence of the antibiotic). Presumably, the substrate
normally binds first in the preinsertion state (Figure 6.38b),
and this allows the enzyme to examine it for correct basepairing and for the correct sugar (ribose vs. deoxyribose)
before it switches to the insertion state (Figure 6.38a),
where it can be examined again for correct base-pairing
with the template base. Thus, the two-state model helps to
explain the fidelity of transcription.
The great similarity in structure of the active site
among RNA polymerases from all kingdoms of life suggests that all should use the same mechanism of substrate
addition, including the two-state model described here.
However, as we will see in Chapter 10, investigators of the
yeast RNA polymerase have described a two-state model
that includes an “entry state” that differs radically from
the preinsertion state described here. The substrate in the
“entry site” is essentially upside down with respect to the
substrate in the insertion state. Clearly, in such a position,
it cannot be checked for proper fit with the template base.
Vassylyev and colleagues do not dispute the existence of
the entry site, but postulate that, if it exists, it must represent a third state of the entering substrate, which must
precede the preinsertion state.
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Chapter 6 / The Mechanism of Transcription in Bacteria
(a)
SUMMARY Structural studies of the elongation
complex involving the Thermus thermophilus
RNA polymerase have revealed the following
features: A valine residue in the b9 subunit inserts
into the minor groove of the downstream DNA.
In this position, it could prevent the DNA from
slipping, and it could induce the screw-like motion of the DNA through the enzyme. Only one
base-pair of DNA (at position 11) is melted and
available for base-pairing with an incoming nucleotide, so only one nucleotide at a time can
bind specifically to the complex. Several forces
limit the length of the RNA–DNA hybrid. One of
these is the length of the cavity in the enzyme
that accommodates the hybrid. Another is a hydrophobic pocket in the enzyme at the end of the
cavity that traps the first RNA base displaced
from the hybrid. The RNA product in the exit
channel assumes the shape of one-half of a doublestranded RNA. Thus, it can readily form a hairpin to cause pausing, or even termination of
transcription. Structural studies of the enzyme
with an inactive substrate analog and the antibiotic streptolydigin have identified a preinsertion
state for the substrate that is catalytically inactive, but could provide for checking that the substrate is the correct one.
Topology of Elongation Does the core, moving along
the DNA template, maintain the local melted region created during initiation? Common sense tells us that it
does because this would help the RNA polymerase
“read” the bases of the template strand and therefore
insert the correct bases into the transcript. Experimental
evidence also demonstrates that this is so. Jean-Marie
Saucier and James Wang added nucleotides to an open
promoter complex, allowing the polymerase to move
down the DNA as it began elongating an RNA chain,
and found that the same degree of melting persisted.
Furthermore, the crystal structure of the polymerase–
DNA complex shows clearly that the two DNA strands
feed through separate channels in the holoenzyme, and
we assume that this situation persists with the core polymerase during elongation.
The static nature of the transcription models presented in Chapter 6 is somewhat misleading. If we could
see transcription as a dynamic process, we would observe the DNA double helix opening up in front of the
moving “bubble” of melted DNA and closing up again
behind. In theory, RNA polymerase could accomplish
this process in two ways, and Figure 6.39 presents both
of them. One way would be for the polymerase and the
growing RNA to rotate around and around the DNA
(b)
Figure 6.39 Two hypotheses of the topology of transcription of
double-stranded DNA. (a) The RNA polymerase (pink) moves
around and around the double helix, as indicated by the yellow arrow.
This avoids straining the DNA, but it wraps the RNA product (red)
around the DNA template. (b) The polymerase moves in a straight
line, as indicated by the yellow arrow. This avoids twisting the RNA
product (red) around the DNA, but it forces the DNA ahead of the
moving polymerase to untwist and the DNA behind the polymerase to
twist back up again. These two twists, represented by the green
arrows, introduce strain into the DNA template that must be relieved
by topoisomerases.
template, following the natural twist of the double-helical
DNA, as transcription progressed (Figure 6.39a). This
would not twist the DNA at all, but it would require
considerable energy to make the polymerase gyrate that
much, and it would leave the transcript hopelessly
twisted around the DNA template, with no known enzyme to untwist it.
The other possibility is that the polymerase moves in a
straight line, with the template DNA rotating in one direction ahead of it to unwind, and rotating in the opposite
direction behind it to wind up again (Figure 6.39b). But
this kind of rotating of the DNA introduces strain. To visualize this, think of unwinding a coiled telephone cord, or
actually try it if you have one available. You can feel (or
imagine) the resistance you encounter as the cord becomes
more and more untwisted, and you can appreciate that you
would also encounter resistance if you tried to wind the
cord more tightly than its natural state. It is true that the
rewinding of DNA at one end of the melted region creates
an opposite and compensating twist for the unwinding at
the other. But the polymerase in between keeps this compensation from reaching across the melted region, and the
long span of DNA around the circular chromosome insulates the two ends of the melted region from each other the
long way around.
So if this second mechanism of elongation is valid, we
have to explain how the strain of unwinding the DNA is
relaxed. As we will see in Chapter 20 when we discuss DNA
replication, a class of enzymes called topoisomerases can
introduce transient breaks into DNA strands and so relax
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6.4 Elongation
this kind of strain. We will see that strain due to twisting a
double-helical DNA causes the helix to tangle up like a
twisted rubber band. This process is called supercoiling, and
the supercoiled DNA is called a supercoil or superhelix.
Unwinding due to the advancing polymerase causes a compensating overwinding ahead of the unwound region.
(Compensating overwinding is what makes it difficult to
unwind a coiled telephone cord.) The supercoiling due to
overwinding is by convention called positive. Thus, positive
supercoils build up in front of the advancing polymerase.
Conversely, negative supercoils form behind the polymerase.
One line of evidence that directly supports this model of
transcription comes from studies with topoisomerase mutants that cannot relax supercoils. If the mutant cannot
relax positive supercoils, these build up in DNA that is
being transcribed. On the other hand, negative supercoils
accumulate during transcription in topoisomerase mutants
that cannot relax that kind of superhelix.
SUMMARY Elongation of transcription involves
the polymerization of nucleotides as the RNA
polymerase travels along the template DNA. As it
moves, the polymerase maintains a short melted
region of template DNA. This requires that the
DNA unwind ahead of the advancing polymerase
and close up again behind it. This process introduces strain into the template DNA that is relaxed
by topoisomerases.
Pausing and Proofreading The process of elongation is
far from uniform. Instead, the polymerase repeatedly
pauses, and in some cases backtracks, while elongating an
RNA chain. Under in vitro conditions of 218C and 1 mM
NTPs, pauses in bacterial systems have been found to be
very brief: generally only 1–6 sec. But repeated short pauses
significantly slow the overall rate of transcription. Pausing
is physiologically important for at least two reasons: First,
it allows translation, an inherently slower process, to keep
pace with transcription. This is important for phenomena
such as attenuation (Chapter 7), and aborting transcription
if translation fails. The second important aspect of pausing
is that it is the first step in termination of transcription, as
we will see later in this chapter.
Sometimes the polymerase even backtracks by reversing
its direction and thereby extruding the 39-end of the growing
transcript out of the active site of the enzyme. This is more
than just an exaggerated pause. For one thing, it tends to last
much longer: 20 sec, up to irreversible arrest. For another, it
occurs only under special conditions: when nucleotide concentrations are severely reduced, or when the polymerase
has added the wrong nucleotide to the growing RNA chain.
In the latter case, backtracking is part of a proofreading
process in which auxiliary proteins known as GreA and
155
GreB stimulate an inherent RNase activity of the polymerase
to cleave off the end of the growing RNA, removing the
misincorporated nucleotide, and allowing transcription to
resume. GreA produces only short RNA end fragments 2–3 nt
long, and can prevent, but not reverse transcription arrest.
GreB can produce RNA end fragments up to 18 nt long, and
can reverse arrested transcription. We will discuss the analogous proofreading mechanism in eukaryotes in greater detail
in Chapter 11.
One complication to this proofreading model is that the
auxiliary proteins are dispensable in vivo. And yet one
would predict that mRNA proofreading would be important for life. In 2006, Nicolay Zenkin and colleagues suggested a resolution to this apparent paradox: The nascent
RNA itself appears to participate in its own proofreading.
Zenkin and colleagues simulated an elongation complex
by mixing RNA polymerase with a piece of single-stranded
DNA and an RNA that was either perfectly complementary
to the DNA or had a mismatched base at its 39-end. When
they added Mg21, they observed that the mismatched RNA
lost a dinucleotide from its 39-end, including the mismatched
nucleotide and the penultimate (next-to-last) nucleotide.
This proofreading did not occur with the perfectly matched
RNA. The fact that two nucleotides were lost suggests that
the polymerase had backtracked one nucleotide in the mismatched complex. And this in turn suggested a chemical
basis for the RNA-assisted proofreading: In the backtracked
complex, the mismatched nucleotide, because it is not basepaired to the template DNA, is flexible enough to bend back
and contact metal II, holding it at the active site of the enzyme. This would be expected to enhance phosphodiester
bond cleavage, because metal II is presumably involved in
the enzyme’s RNase activity. In addition, the mismatched
nucleotide can orient a water molecule to make it a better
nucleophile in attacking the phosphodiester bond that links
the terminal dinucleotide to the rest of the RNA. Both of
these considerations help to explain why the mismatched
RNA can stimulate its own cleavage, while a perfectly
matched RNA cannot.
SUMMARY RNA polymerase frequently pauses, or
even backtracks, during elongation. Pausing allows
ribosomes to keep pace with the RNA polymerase,
and it is also the first step in termination. Backtracking aids proofreading by extruding the 39-end of the
RNA out of the polymerase, where misincorporated
nucleotides can be removed by an inherent nuclease
activity of the polymerase, stimulated by auxiliary
factors. Even without these factors, the polymerase
can carry out proofreading: The mismatched nucleotide at the end of a nascent RNA plays a role in
this process by contacting two key elements at the
active site: metal II and a water molecule.
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