J Bacteriol, June 1998, p. 3019-3025, Vol. 180, No. 12
Department of Biochemistry, Case Western
Reserve University, Cleveland, Ohio 44106-4935,1
and
Departments of Biochemistry2 and
Chemistry,3 University of Wisconsin,
Madison, Wisconsin 53706
Initiation of transcription is a
complicated process involving several different phases: promoter
location by RNA polymerase, formation of a competent initiation
complex, synthesis of the initial phosphodiester bonds, and movement of
RNA polymerase from the promoter as it enters the elongation phase; as
shown below, further subdivisions may be warranted. The large number of
different steps has afforded a multitude of focal points at which
control of the process could be exerted. Many of these have been
exploited by the bacterial cell, as exemplified by the rich variety of
regulatory mechanisms that have been uncovered (Fig.
1). To arrive at a better understanding
of both the sequence of events in the initiation of RNA synthesis and
the control of the process, it is important to identify the
intermediate species involved. As detailed in a recent review
(68), a major challenge is not only to correlate kinetically
observed intermediates with those that can be trapped and characterized
but also to put all intermediates in the context of structural
information as it becomes available.
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
MINIREVIEW
RNA Polymerase-Promoter Interactions: the Comings
and Goings of RNA Polymerase
![]()
INTRODUCTION
Top
Introduction
Concluding Remarks
References

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FIG. 1.
Summary of the intermediates in the process of
initiation of RNA synthesis for which structural and/or kinetic
evidence has been obtained; see the text for details. The complexes are
shown in cartoon form at the top with a descriptive notation below. In
the designation of structurally characterized complexes, R stands for
RNA polymerase, P stands for promoter DNA, and c and o indicate closed
and open complexes, respectively (where strand separation has not or
has occurred, respectively). I1 and I2 are the
kinetically significant intermediates, shown by DNA footprinting to be
RPc2-like complexes. AP stands for abortive RNA product,
and TC stands for the transcribing complex in elongation mode. Also
indicated are examples of agents or conditions that can affect (inhibit
or activate) the interconversion of intermediates; some of these (CRP,
cI, and arc) have different effects, depending on the promoter or RNA
polymerase. Some are E. coli proteins: the lac
repressor (LacR [78]), CRP (which activates different
processes, depending on the promoter [18, 34, 52, 55,
62]), and elongation factors GreA and GreB (which can affect
promoter escape in vitro [36]). Phage-encoded proteins
cI (lambda) and arc (P22) display polymerase- or promoter-dependent
effects: cI inhibits the binding of a mutant RNA polymerase (mut RNAP)
(49) but affects I1-I2 isomerization
with wild-type RNA polymerase (31); arc represses wild-type
promoters by slowing I1-I2 isomerization but
activates a consensus promoter mutant by accelerating clearance
(85). Rifampin is an antibiotic which targets the
subunit of RNA polymerase; heparin binds and inactivates free RNA
polymerase. The latter two agents have been useful tools in the study
of RNA polymerase-promoter interactions. Growth rate control
(20) and reiterative (Reit.) RNA synthesis (synth.)
(50) are responsive to NTP levels (see text).
Here we will emphasize mechanistic aspects of the interaction of RNA
polymerase containing the initiation factor
70 (which is
responsible for the vast majority of initiation events in
Escherichia coli) with promoters to form
initiation-competent complexes and on the dissolution of these
complexes when the RNA polymerase leaves the promoter in the course of
chain initiation. Possible intermediates for open-complex formation at
various promoters have been identified as well (68; cf. Fig.
1). Here we focus on the PR promoter of bacteriophage
. It is likely that open-complex formation at other promoters
involves a similar order of events, so that the principles which are
emerging for PR should be generally valid. Following a
brief overview of RNA polymerase and its promoters, we review the
following sequential events in the initiation of transcription:
promoter location, initial reversible binding of RNA polymerase,
conformational changes in RNA polymerase, conformational changes in
DNA, binding of nucleoside triphosphate (NTP) to the functional RNA
polymerase-promoter complex, and nonproductive and productive
initiation of RNA synthesis.
| |
RNA POLYMERASE AND PROMOTERS |
|---|
The initiating RNA polymerase holoenzyme (E
70) has
a molecular weight of about 4.5 × 105 and contains
five subunits with the stoichiometry
2
'
(see Fig. 2). The
subunit is bound relatively weakly to the rest of the
enzyme (the core polymerase, E);
is responsible for specific promoter recognition by RNA polymerase but is released during the
initiation process. The core polymerase then continues to catalyze
phosphodiester bond formation between the growing RNA chain and the
next NTP, whose identity is specified by the sequence of the template
strand. After chain termination and dissociation of the ternary complex
of the core RNA polymerase, RNA, and DNA, the released core can again
bind a
subunit and start a new cycle of RNA synthesis (8,
93). The Escherichia coli genome encodes multiple
factors, of which
70 (named for its molecular weight of
70,000; it is also referred to as
D) is by far the most
abundant. The
subunit is an important determinant of the
sequence-specific recognition of promoter DNA. Through the use of
different
factors, RNA polymerase can be targeted to promoters with
different sequences (15, 33, 47, 51, 68, 87, 97, 101).
We will consider almost exclusively those promoters recognized by the
E
70 holoenzyme and will therefore use the term RNA
polymerase for this particular form. Four important promoter elements
can be distinguished: two hexamers centered at or near positions
10 and
35 upstream from the transcription start site (designated by
their locations as the
10 and
35 regions), the spacer DNA separating them, and a region between
40 and
60 (the UP element) (see Fig. 2). Various compilations of promoter sequences recognized by
E
70 have established the importance of the
35 and
10
regions (29, 32). The consensus sequences of these two
regions as read on the nontemplate strand are, respectively,
TTGACA and TATAAT (Fig. 2). With a few exceptions (19,
42), the general rule holds that the greater the similarity of
the
10 and
35 regions to their consensus sequence, the better the
promoter functions in vitro, as well as in vivo (19, 29, 32, 42,
68); in this regard, well-conserved base pairs may be of greater
importance than the less-conserved ones (57). The ability of
RNA polymerase to discriminate between promoters with different
sequences at the
10 and
35 regions has allowed the unregulated (or
basal) rate of transcription of various genes to be set at different levels. A consensus length of 17 bp has been established for the spacer
between the
10 and
35 regions. Promoters with such a spacer length
have been found to be more active in vitro, as well as in vivo, than
those with shorter or longer spacers (2, 58, 86). Quite
recently, a very A+T-rich region between positions
40 and
60 (the
UP element) was recognized in some promoters as an additional important
determinant of promoter activity. This region is contacted by the
subunit (5, 22, 48, 67, 74). In an initiation-competent
complex between RNA polymerase and promoter DNA, the two strands of a
12- to 15-bp region from about the middle of the
10 element to just
past the start site (+2 or +3) become more reactive to footprinting
agents (43, 68, 76, 82, 90, 100); therefore, such a complex
is referred to as the "open" promoter complex.
|
A subclass of E. coli promoters function quite well without
a recognizable
35 region or the involvement of any activating proteins. Such promoters have been found to have an "extended
10
region" with the sequence TGNTATAAT (15, 56, 75,
96) (Fig. 2). Open-complex formation at these promoters proceeds
at rates comparable to those for promoters with consensus
10 and
35
regions (14a) and appears to be less temperature dependent than open-complex formation at promoters with
35 regions
(56). It was recently shown that a region of
70 not previously identified as conserved is implicated
in recognition of the upstream TG sequence (4).
| |
PROMOTER LOCATION |
|---|
There has been much speculation concerning the kinetic pathway along which RNA polymerase locates promoter sequences embedded in nonspecific DNA. The RNA polymerase holoenzyme has a measurable affinity for nonspecific sites (14), but whether this mode of binding plays a role in facilitating promoter location (41, 83, 84) is still uncertain. For several other proteins, including the lac repressor and the EcoRI restriction enzyme (3, 92, 95), considerable evidence has been obtained for acceleration of target site location through facilitated diffusion mediated by nonspecific binding. Possible mechanisms by which this is accomplished include one-dimensional diffusion along the DNA, "hopping" (successive association and dissociation within the domain of the DNA molecule), and intersegment transfer between regions of DNA that are close in space but separated by long DNA sequence stretches (reviewed in reference 94). Direct evidence for translocation of RNA polymerase along DNA fragments has been obtained (41), but it is unclear whether this was by one-dimensional diffusion or hopping. In other reports, linear diffusion of RNA polymerase along DNA was inferred, but it was not clearly established that the diffusion played a role in the acceleration of promoter location of the enzyme (70, 83, 84). Intersegment transfer would require the availability of two separate DNA binding sites which transiently are simultaneously occupied during the transfer process. The existence of two such sites has not been demonstrated for the RNA polymerase protomer.
At the strongest known promoters (those for the synthesis of rRNA), in
vivo chain initiation occurs at the rate of about 1/s (68),
which is thus the lower limit for the rate of binding of RNA polymerase
to the promoter. As the free RNA polymerase concentration in the cell
has been estimated to be about 30 nM (54), a crude estimate
of the association rate constant is the reciprocal of this, or about
3 × 107 M
1 s
1. Rates of
up to 3 × 108 M
1 s
1 have
been observed in vitro (6, 67) for promoters contained on
restriction fragments (typically, 200 to 1,000 bp, long enough to
benefit from facilitated diffusion). These rates do not exceed reasonable estimates for the rate of a reaction limited by
three-dimensional diffusion, i.e., direct interaction of the reactants
without nonspecific binding: 108 to 109
M
1 s
1 (68). In addition,
comparable or faster rates have been observed by using fragments of
synthetic DNA too short (90 bp) for facilitated diffusion to
significantly accelerate the target search (12a). Finally,
accelerated diffusion is not expected to lead to a rate enhancement for
the majority of promoters, where the initial (closed) promoter complex
is in rapid equilibrium with free promoter and RNA polymerase prior to
proceeding down the pathway shown in Fig. 1. Therefore, McClure's 1985 conclusion (54) for RNA polymerase is still valid today:
"... the experiments that show the enzyme can slide do not
establish that it must do so as a rate enhancement mechanism."
| |
INITIAL REVERSIBLE BINDING OF RNA POLYMERASE TO PROMOTER SITES |
|---|
Some time ago, it was proposed (24) that sequences in
the
35 region would affect the initial binding of RNA polymerase to
the promoter and that those in the
10 region would affect the
isomerization to the open complex. Thus, mutations in the two promoter
elements would affect different steps (30, 80, 81). The
model fell out of favor when kinetic studies failed to find consistent
differences in the way base changes in either region affected the
kinetics of open-complex formation (e.g., see references
54 and 91). Current experimental
evidence indicates that the initial contacts between RNA polymerase and
the promoter to form the first double-stranded, or closed, intermediate
(RPc1; Fig. 1) probably do involve the
35 region, but the
role of the
10 region remains unclear. The
35 region remains double
stranded throughout the process of open-complex formation. It has been demonstrated that polypeptides of
70 can recognize both
35 and
10 sequences as double-helical DNA, although only the
upstream half of the
10 region appears to be important in this
respect (15-17). Finally, the RNA polymerase holoenzyme has
been observed to bind single-stranded oligodeoxynucleotides bearing the
10 sequence of the nontemplate strand (37, 53, 71,
and see below) with greater affinity than double-stranded DNA spanning
the same region (37). Perhaps initial recognition of
10
sequences involves the upstream half of the region, approximately coinciding with the part of the region remaining double helical in an
open complex.
From studies of several promoters at low temperatures (10°C and
below), the equilibrium constant (K1) for the initial
binding of RNA polymerase to form a closed complex has been estimated to be about 107 to 108 M
1 in
buffer containing 0.1 M monovalent cation and 0.01 M MgCl2. DNase I footprinting experiments with this low-temperature complex (RPc1) at some promoters indicate that RNA polymerase
contacts the DNA from
55 to
5. At somewhat higher temperatures, a
second closed RNA polymerase-promoter complex is experimentally
detectable by virtue of a downstream extension in the footprint; this
intermediate has been designated RPc2 (reviewed in
reference 68). These and other complexes which have
been structurally characterized by probing with DNase I and
KMnO4 are indicated in the first line below the cartoons in
Fig. 1.
| |
REVERSIBLE CONFORMATIONAL CHANGES IN RNA POLYMERASE |
|---|
Two intermediates (designated I1 and I2 in Fig. 1) are kinetically significant; their interconversion is the rate-limiting step in open-complex formation at all temperatures, in both the association and dissociation directions (78a). A key question in recent years has involved the correlation of the kinetic and structural intermediates. To address this question, I1 and I2 were trapped by using thermodynamic conditions under which each was predicted to accumulate. It was found that both intermediates are closed complexes with extended footprints; in this regard, they are both RPc2-like complexes (11a).
Analysis of the thermodynamics of the conversion of I1 to I2 indicates that a very large amount of the nonpolar surface is buried, leading to the proposal that this transition involves a major conformational change in RNA polymerase (72, 73, 78a). Although the nature of this conformational change has yet to be elucidated, one possibility is that the conversion of I1 to I2 is the step at which a jawlike structure in RNA polymerase closes around the DNA (Fig. 1). This structure is believed to contain the active site of the polymerase and has been shown to be open in the holoenzyme (and by inference in the early steps of promoter recognition) (12) but closed in the core polymerase (and, by inference, at the end of the long chain of events that results in initiation of RNA synthesis) (66). A similar hypothesis has been advanced by Heumann's group (77), although the case has also been made for jaw closing at a later stage, in response to RNA synthesis (64).
| |
CONFORMATIONAL CHANGES IN THE DNA |
|---|
In the process of open-complex formation, conformational changes
also take place in promoter DNA; of these, the most salient is the
strand separation extending from the
10 region to past the start site
(7, 10, 11, 43, 76, 82, 90). The process takes place under
conditions in which the double-stranded form of DNA is resistant to
denaturation by about 1 kcal/mol bp, yet no external source of energy
is required to drive the strand separation process. Open complexes
generally are very stable, and their formation is quite fast (see
above). Thus, it seems plausible that RNA polymerase would not only
stabilize the open complex once it was formed but also lower the
activation energy for the strand separation process. It is envisaged
that this is accomplished by RNA polymerase-induced DNA distortions
that destabilize the double-helical form. Such distortions might
include torqueing of the DNA to introduce an unwinding twist and DNA
bending across the promoter DNA. The most compelling evidence for the
former (recently reviewed [13]) is derived from
studies of the effects of DNA supercoiling on promoter utilization. The
results can be interpreted to indicate that RNA polymerase unwinds
promoter DNA by as much as half a turn early in the process of
open-complex formation (1, 88). It seems likely that the
untwisting torque would be applied directly across the region of strand
separation. While this possibility has previously been explicitly
suggested (88), to the best of our knowledge, there is no
direct experimental data to support it; some studies suggest a
rotational distortion across the spacer DNA separating the
10 and
35 regions (13). Strong evidence in support of promoter
DNA bending by RNA polymerase has been obtained from the visualization
of the complexes by atomic force microscopy (69), as well as
from gel shift experiments (35). The results of footprinting
and chemical probing experiments suggest that the DNA is wrapped around
RNA polymerase over a region of 70 to 80 bp (11, 46, 61);
reviewed in references 23 and
68). Whether the wrapping or bending is required to
lower the stability of the helix in the region where strand opening occurs has not been established experimentally.
Once formed, the open complex is presumably stabilized by
interactions between single-stranded DNA and the RNA polymerase. Indeed, Roberts and coworkers (53, 71; see also
reference 37) have recently described the
sequence-specific recognition by the holoenzyme of nontemplate strand
10 sequences contained on small single-stranded
oligodeoxynucleotides. The rate of binding of such molecules is
an order of magnitude slower than that for open-complex formation
at a promoter with a similar
10 region and a consensus
35 (12a). This observation is consistent with the notion
that for the single-stranded DNA to interact with the RNA polymerase,
the latter has to undergo a change in conformation, which would be
facilitated by the binding of double-helical promoter DNA but not of a
short, single-strand oligomer.
Completion of the process of RNA polymerase-induced strand opening is
dependent on Mg2+ (89, 90, 100). The complex
formed in the absence of Mg2+ has been designated
RPo1 (Fig. 1); it shows base pair opening extending from
the middle of the
10 region to bp
1, just upstream of the start
site (90). It is thought to be an intermediate in the
formation of the functional open complex RPo2 (Fig. 1), formed in the presence of Mg2+, in which opening extends
from
12 to +2. An interesting question involves the order in which
DNA bases become unpaired to form RPo2 (10, 13):
is the process cooperative, or can partially melted intermediates be
identified from which the progression of strand opening can be deduced?
Current experimental evidence points to initiation of strand opening in
the
10 region and then propagation downstream toward the start site.
This conclusion is based primarily on chemical probing experiments
carried out on complexes detected at different temperatures
(10), but recent fast-kinetic studies also are consistent
with such a model (12a).
| |
NTP BINDING TO DRIVE FORMATION OF TERNARY COMPLEXES |
|---|
After strand opening, the template strand becomes accessible to
the NTPs so that it can (by Watson-Crick base pairing) specify the
sequence of the RNA to be synthesized. The kinetic stability of the
open complex can be enhanced (i.e., the rate of dissociation is
diminished) by the binding of the initiating NTP without requiring its
hydrolysis or formation of a phosphodiester bond (20). The rrnB P1 promoter for the synthesis of rRNA forms an unstable open complex even at 37°C (25, 65); most likely, this is due to both the nonconsensus spacer length between the
10 and
35 regions (16 instead of 17 bp) and the presence of an abundance of G+C base
pairs in the melted region between
10 and +1. Kinetic stabilization of the open complex by a high concentration (in the millimolar range)
of ATP (the initiating NTP) at this promoter is observed in vitro
(20). In agreement with this result, it was found that the
activity of rrnB P1 in vivo is controlled by the intracellular concentration of ATP, the level of which was shown to vary with the
growth rate of the cell. High levels of ATP (as found in fast-growing cells) result in greatly enhanced synthesis of rRNA, presumably by
kinetically stabilizing the open complexes at the promoter and allowing
enough time for formation of the first phosphodiester bond to occur
(20). These observations go a long way toward solving the
long-standing problem of growth rate control of rRNA synthesis
(21).
| |
NONPRODUCTIVE AND PRODUCTIVE INITIATION OF RNA SYNTHESIS |
|---|
Ternary complexes of promoter, RNA polymerase, and NTP(s) are poised to initiate productive RNA synthesis, but after formation of the first phosphodiester bond, several hurdles still need to be overcome before the complex is committed to productive RNA synthesis. It has been known since the early 1980s that a significant fraction of the complexes engages in abortive RNA synthesis in which short (<10-nt) RNA transcripts are made and released in a repetitive manner (9, 26, 59). As a result of these studies, abortive initiation has been regarded as a phase in the process of productive chain initiation. After each catalytic step, a stochastic event would determine whether the nascent RNA was released or further elongated, until a critical length (about 10 nt) was reached, whereupon the complex became committed to productive RNA synthesis. Based on recent results obtained with the PR promoter, an alternative possibility has been suggested. From a homogeneous population of RNA polymerases, each molecule has a certain probability of becoming irreversibly trapped in carrying out abortive synthesis or of escaping this fate and entering the elongation mode (45). It is unclear how the two different models can be reconciled.
Another nonproductive mode of initiation involves a process termed "reiterative RNA synthesis," which was found to play a role in the UTP concentration-dependent regulation at the pyrBI operon (40). Here, the first six nucleotide residues of the nascent RNA are AAUUUG. At high UTP concentrations, RNA polymerase reiteratively transcribes bases 3 to 5 in the template strand to synthesize RNA products containing 30 or more U residues. While engaged in this process, productive initiation at the promoter is blocked and expression of the operon is inhibited. At low UTP concentrations, extension by G successfully competes with the reiterative addition of U and this mode is bypassed. This is an additional example of the regulation of productive initiation of RNA synthesis by levels of NTP. Two important differences from the rrnB P1 promoter are that the regulation is by a noninitiating NTP and that at high levels it promotes a decrease in initiation of transcription. Several other cases of reiterative RNA synthesis have been described (39, 50), including some at promoters bearing mutations which introduce a string of A residues in the template strand near the position of initiation of RNA synthesis (38, 99).
Once an RNA chain of eight or nine nucleotide residues has been
synthesized,
factor is released (28, 44); at a similar length of the nascent RNA chain (10 nucleotides [44]),
RNA polymerase leaves the promoter and becomes committed to productive
chain elongation. The physical movement of the RNA polymerase away from the promoter (as assayed, for instance, by the reclosure of the region
of strand separation between
10 and +2) is referred to as "promoter
clearance." The rates of clearance at various promoters vary
significantly (19, 79, 85). It is thought that there is a
direct relationship between the strengths of the RNA
polymerase-promoter contacts and the rate with which promoter clearance
is achieved (19, 42). The fact that
factor is
responsible for many of the RNA polymerase-promoter contacts may then
explain the coincidence of
factor release and the commitment to
elongation, both occurring when the length of the nascent RNA is about
10 nucleotides.
Clearly, the RNA polymerase-promoter complex is held together by
contacts in addition to those involving
factor, as the complex does
not fall apart upon
factor release. Promoter clearance is dependent
upon the sequence of the region downstream of the start site (19,
26, 42). It is unknown whether these differences in sequence
affect the downstream contacts between the RNA polymerase and DNA or
whether they are exerted at the level of RNA synthesis or structure.
Based on available evidence, either of these possibilities could be
envisaged. Contacts between the
' subunit and the downstream DNA
have been found to stabilize the elongating complex (63). However, whether similar contacts may be formed during initiation and
whether they would be sequence dependent has not been established.
| |
CONCLUDING REMARKS |
|---|
|
|
|---|
Substantial progress has been made in understanding the
interaction of RNA polymerase with promoters in prokaryotes. The
various intermediates which have been identified kinetically or
structurally, as well as some agents or conditions which can affect
their interconversion, are shown in Fig. 1 (details concerning the
effectors can be found in the legend). Several aspects of the scheme in
Fig. 1 remain unclear, however, and require further experimentation for
their elucidation. The nature of the conformational change that occurs during the transition from I1 to I2 is a
subject of great importance. The hypothesis that this step involves the
closing of the jaw on RNA polymerase needs to be tested. How RNA
polymerase weakens the base pairing interactions of DNA in order to be
able to orchestrate the separation of the strands also remains to be
established, as does the precise site of initiation of strand
separation and the involvement of specific amino acid residues of RNA
polymerase in this process. It is necessary to pinpoint the precise
amino acid residue-nucleotide interactions which occur in the various intermediate initiation complexes to develop a clearer picture of the
early steps of transcription. Also, it needs to be determined what
triggers the release of
factor from the initiating RNA polymerase.
Finally, the role of NTPs in open-complex stabilization, start site
selection, and the transition from initiation to elongation needs to be
further explored.
| |
ACKNOWLEDGMENTS |
|---|
We thank P. de Boer, P. Rather, and D. Setzer, as well as members of the deHaseth and Record groups, for their comments and suggestions.
Our research is supported by N.I.H. grants GM 31808 (P.L.H.) and GM 23467 (M.T.R.).
| |
FOOTNOTES |
|---|
* Corresponding author. Mailing address: Department of Biochemistry, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106-4935. Phone: (216) 368-3684. Fax: (216) 368-4544. E-mail: pld2{at}po.cwru.edu.
| |
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