![]()
|
|
||||||||
J. Biol. Chem., Vol. 281, Issue 16, 10653-10656, April 21, 2006
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
From the Laboratory of DNA Replication, Howard Hughes Medical Institute, The Rockefeller University, New York, New York 10021
| INTRODUCTION |
|---|
|
|
|---|
DNA polymerase III (pol III)2 functions in the context of a multiprotein apparatus called DNA pol III holoenzyme (reviewed in (35)). pol III holoenzyme contains 10 different proteins, which assort into three major functional units: 1) pol III core, 2) the
sliding clamp, and 3) the
/
complex clamp loader. The holoenzyme contains two copies of the pol III core, which are connected by the attachment to one clamp loader. The holoenzyme functions within the context of a dynamic replisome containing pol III holoenzyme, a hexameric DnaB helicase, DnaG primase, and SSB. During replisome function, contacts between these proteins are in a constant state of change. This review briefly summarizes the architecture and dynamic behavior of the E. coli replisome.
| The Clamp and Clamp Loader |
|---|
|
|
|---|
sliding clamp is a homodimer in the shape of a ring, which encircles DNA (Fig. 1A). The
clamp slides on DNA and binds the pol III core, thereby acting as a mobile tether and converting the normally distributive pol III core into a highly processive and rapid polymerase capable of incorporating 5001,000 nucleotides per second (Fig. 1C). The crystal structure of the
dimer reveals a 6-fold pseudo symmetry that arises from a domain that is repeated three times in the monomer giving the dimer a 6-fold appearance (6). The eukaryotic PCNA clamp and phage T4 gp45 clamp are also six-domain rings, but the monomeric unit contains only two domains and trimerizes to form a six-domain ring (7).
Clamps do not self-assemble onto DNA but require a multiprotein clamp loader, which harnesses the energy of ATP hydrolysis to open and close the clamp around DNA (Fig. 1B). The E. coli
complex clamp loader contains five subunits that are essential for clamp loading activity, three
protomers and one copy each of
and
'. The small
and
subunits are not required for clamp loading, but they stabilize the clamp loader and stimulate its activity at elevated ionic strength.
The
,
, and
' clamp loading subunits are members of the AAA+ family (ATPases associated with a variety of cellular functions) (8). The subunits consist of three domains, and a AAA+ region of homology is localized to the first two domains (9). Many AAA+ proteins are homohexamers arranged in a symmetric circle (10, 11). The
3
' pentamer forms an asymmetric circle, and there is a gap in place of the "missing sixth subunit" (Fig. 1B) (12). The C-terminal domains form an uninterrupted collar that ties the pentamer together.
The
clamp docks onto the AAA+ domains of
3
' (Fig. 1, B and C). This placement comes from the structure of
-
1 (13). Replacing
in
3
' with
-
1 indicates that
interacts with the other subunits, confirmed biochemically (14). When
binds to
it opens the clamp (15, 16). However, ATP binding is required for the
complex to adopt a conformation allowing it to interact with
(17), and thus the unliganded crystal structure is in an inactive conformation. The
-
1 crystal structure reveals that the
dimer is under tension, and when the interface is distorted by
, the clamp springs open (13).
The AAA+ domains of the
complex are arranged in a spiral (7, 12). Structural studies of the Saccharomyces cerevisiae RFC clamp loader-ATP
S-PCNA complex also show this spiral arrangement. The RFC-ATP
S spiral has a steeper helical pitch than
complex and closely matches the pitch of B-DNA for DNA interaction (18). DNA fits inside the clamp loader where the AAA+ domains form a central chamber and two
helices on each subunit track the minor groove of DNA modeled inside. Several polar and basic side chains on and near these helices are conserved in prokaryotic and eukaryotic clamp loaders. Mutations of these conserved residues in either
or
' significantly reduce ability of
complex to bind DNA (19).
| The Open Clamp Is a Spiral Lockwasher |
|---|
|
|
|---|
3
' and RFC subunits is compatible with a right-handed opening of the clamp, thereby allowing it to dock onto the other clamp loading subunits. Indeed, molecular simulations of PCNA indicate that it springs open out-of-plane with a strong tendency to form a right-handed helix (21). In addition, electron microscopic reconstruction of an archael RFC-PCNA-ATP
S-DNA complex provides direct visual evidence for an open clamp in a right-handed helix (22). DNA is guided from the central chamber of the clamp loader into the clamp docked below (see Fig. 1C). DNA binding is followed by hydrolysis of ATP to eject the clamp loader. Clamp loader ejection is necessary as the pol III core binds the same face of
as the clamp loader, and only one of these complexes can interact with
at a given time (23, 24).
Clamp loader structures suggest how specificity for a primed site is gained (7, 12). Duplex DNA enters into the open clamp and the central chamber of the clamp loader through the gap between
and
'. However, the C-terminal domains of the clamp loader form a tight collar with no interruption and thus act as a cap that DNA cannot penetrate. To fit into the clamp loader, DNA must make a sharp bend at the cap and exit out the gap in the side of the clamp loader. A continuous duplex cannot bend abruptly and thus is unable to fit, but ssDNA at a primed site provides flexibility for bending, and thus this structure can be accommodated (e.g. Fig. 1C).
| pol III Holoenzyme and Architecture of the Replisome |
|---|
|
|
|---|
subunit (see Fig. 2A; reviewed in Refs. 35).
is encoded by the same gene as
, but
lacks the C-terminal 24 kDa of
due to truncation by a translational frameshift. The C-terminal region unique to
, referred to as
c, binds to the pol III core. The pol III core consists of three subunits,
(DNA polymerase),
(3'-5'-exonuclease), and
. Within the holoenzyme two copies of
replace two
subunits to form a 
2
'
clamp loader, which in turn binds two pol III cores for leading and lagging strand replication (Fig. 2A). The C termini of the
subunits protrude from the collar and contain many prolines and polar residues. Hence, the
c region is likely connected to the
clamp loading domains by a flexible linker to form a pol III holoenzyme containing two cores with one clamp loader suspended from flexible linkers between them. The single clamp loader can load
clamps onto the leading and lagging strands for both DNA pol III cores (25).
The
c region connects pol III holoenzyme to the replicative helicase, DnaB (26). DnaB is a circular homohexamer, similar to the homohexameric helicases of the T4 and T7 phages (2730). DnaB helicase encircles ssDNA and tracks along it during unwinding action; its direction of movement corresponds to the direction of the replisome provided DnaB encircles the lagging strand (Fig. 2B). Unwinding is achieved by steric exclusion in which one strand is excluded from the inside channel, while the tracking strand is retained on the inside of the ring, thus forcing the duplex apart as DnaB moves (3032).
It is hypothesized that pol III holoenzyme is functionally asymmetric since leading and lagging strand processes are so different, and evidence for asymmetry has been gleaned from studies using ATP
S (3335). pol III holoenzyme is also structurally asymmetric, as defined by the clamp loader, which has odd numbers of subunits (36, 37). The DnaB helicase adds asymmetry to the replisome due to its placement on the lagging strand and has been demonstrated to impose asymmetric function on the two polymerases of pol III holoenzyme (25).
|
|
|
| Replisome Dynamics |
|---|
|
|
|---|
continuously extends DNA in the direction of unwinding. Due to the antiparallel structure of DNA, the lagging strand is chemically extended in the opposite direction of the replication fork. Yet the lagging pol III core must physically move with the fork due to its connection to the leading polymerase and helicase. Hence, the lagging strand must form a loop to accommodate these opposed motions, as hypothesized in the "trombone model" of replication (49). Discontinuous lagging strand fragments are 13 kb in length, and primase initiates each fragment by forming a short RNA primer. Primase must interact with DnaB for activity, which ensures that priming occurs at the replication fork junction (45). Primase acts in a fully distributive fashion (50). As Okazaki fragments are produced every few seconds, primase action is highly dynamic, coming on and off DNA for each priming event.
pol III core held to DNA by a protein ring fits nicely for continuous replication of the leading strand. However, discontinuous synthesis on the lagging strand requires the pol III core to hop from a finished fragment to start the next Okazaki fragment every few seconds. How can pol III core rapidly dissociate from a completed fragment when it is held tight to DNA by a clamp? This dilemma is solved by a processivity switch (51, 52). When the lagging pol III core finishes an Okazaki fragment, it rapidly dissociates from the clamp (and DNA). The clamp loader repeatedly loads
clamps onto RNA primers, which allows the lagging pol III core to reassociate with a new clamp for extension of the next fragment (Fig. 3A). The need for polymerase to be processive, yet rapidly recycle, may underlie one reason that replicative polymerases have evolved to function with clamps.
Separation of pol III from
is achieved by the
c portion of
subunit (53). pol III-
separation is regulated by DNA; primed DNA turns
c off and blocks it from separating pol III from
. But when synthesis is complete,
c separates pol III from
, leaving the clamp on DNA (53). This stoichiometric use of one
clamp for each Okazaki fragment results in a build-up of used clamps on the lagging strand, consistent with the high copy number of
(
300/cell) relative to pol III (1020/cell) (25). The
clamp also interacts with pol I and ligase (54), which are needed to replace the RNA with DNA and seal the finished fragments. Hence, leftover
clamps may mark spots on DNA where these enzymes are needed.
Okazaki fragments outnumber
by about 10-fold, and therefore
clamps must be recycled. The high stability of
on DNA (t
1 h at 37 °C) implies that clamps are actively removed from DNA. The
complex clamp loader is capable of unloading clamps, but the more likely clamp unloader is
, which is in excess over other clamp loading subunits in the cell (55).
cannot load
onto DNA but can open
and is highly active in unloading
from DNA.
| Physically Coupled but Mechanically Uncoupled Polymerases |
|---|
|
|
|---|
Why do the phage replicases strictly couple synthesis of the two strands, while the E. coli replicase does not? One may speculate that rapid duplication of the cellular chromosome is too important to wait for repair of damaged nucleotides on the lagging strand; these can be left behind and repaired later. Due to the smaller size of phage genomes, blocking lesions may only be encountered in a fraction of replicating molecules, allowing chromosomes containing stalled forks to simply be discarded. In this scenario, tight polymerase coupling may act as a fidelity mechanism rather than a strategy for efficient replication.
| Protein Trafficking on Clamps |
|---|
|
|
|---|
clamp interacts with many different proteins, including all five E. coli DNA polymerases, MutS, ligase, and UvrB. These various proteins bind to the hydrophobic site in
to which
binds (54, 6062). E. coli
-polymerase subunit contains two regions that bind
(62). The extreme C-terminal sequence binds the hydrophobic site on
, and the second region is about 200 residues internal to the C terminus (53, 62). Mutations in either region alter the affinity of
for
. The structure of the C-terminal domain of pol IV bound to
also reveals two different sites of interaction with
(63). The extreme C-terminal residues of pol IV bind to one of the hydrophobic pockets in the
dimer, while an internal region of pol IV binds at an edge of the
ring.
The fact that sliding clamps bind numerous proteins has led to speculation that multiple proteins may bind the clamp at the same time (64, 65). In this "toolbelt" hypothesis, the clamp brings together factors that are needed sequentially on DNA. Toolbelt function has been demonstrated for E. coli
in which pol IV and pol III form a ternary complex with
(66). pol IV is a low fidelity Y-family polymerase enabling it to bypass lesions that block the replicase (6769). When pol III stalls, pol IV rapidly gains control of
and the primed template. When the block is relieved, pol III regains the primed template. These protein dynamics limit the action of the low fidelity Pol IV to the locale of the block.
| Concluding Remarks |
|---|
|
|
|---|
coordinate their action on the clamp? What role does DnaB play while it encircles double-strand DNA? How are the forks in a replication factory held together, and what other proteins are present? How do replication enzymes meld their action with repair and recombination processes, such as must occur during replication restart after DNA damage? How do replication fork proteins coordinate with topoisomerases during synthesis and upon termination of the chromosome? These are only some of the many exciting questions that remain for the future. | FOOTNOTES |
|---|
1 To whom correspondence should be addressed: Laboratory of DNA Replication, Howard Hughes Medical Institute, The Rockefeller University, 1230 York Ave., New York, NY 10021. Tel.: 212-327-7252; Fax: 212-327-7253; E-mail: odonnel{at}mail.rockefeller.edu.
2 The abbreviations used are: pol, polymerase; PCNA, proliferating cell nuclear antigen; RFC, replication factor C; ATP
S, adenosine 5'-O-(3-thiotriphosphate); ssDNA, single-stranded DNA. ![]()
| REFERENCES |
|---|
|
|
|---|
This article has been cited by other articles:
![]() |
K. Fujimitsu, M. Su'etsugu, Y. Yamaguchi, K. Mazda, N. Fu, H. Kawakami, and T. Katayama Modes of Overinitiation, dnaA Gene Expression, and Inhibition of Cell Division in a Novel Cold-Sensitive hda Mutant of Escherichia coli J. Bacteriol., August 1, 2008; 190(15): 5368 - 5381. [Abstract] [Full Text] [PDF] |
||||
![]() |
S. Ozaki, H. Kawakami, K. Nakamura, N. Fujikawa, W. Kagawa, S.-Y. Park, S. Yokoyama, H. Kurumizaka, and T. Katayama A Common Mechanism for the ATP-DnaA-dependent Formation of Open Complexes at the Replication Origin J. Biol. Chem., March 28, 2008; 283(13): 8351 - 8362. [Abstract] [Full Text] [PDF] |
||||
![]() |
D. Gawel, P. T. Pham, I. J. Fijalkowska, P. Jonczyk, and R. M. Schaaper Role of Accessory DNA Polymerases in DNA Replication in Escherichia coli: Analysis of the dnaX36 Mutator Mutant J. Bacteriol., March 1, 2008; 190(5): 1730 - 1742. [Abstract] [Full Text] [PDF] |
||||
![]() |
B. Marintcheva, A. Marintchev, G. Wagner, and C. C. Richardson Acidic C-terminal tail of the ssDNA-binding protein of bacteriophage T7 and ssDNA compete for the same binding surface PNAS, February 12, 2008; 105(6): 1855 - 1860. [Abstract] [Full Text] [PDF] |
||||
![]() |
P. McInerney and M. O'Donnell Replisome Fate upon Encountering a Leading Strand Block and Clearance from DNA by Recombination Proteins J. Biol. Chem., August 31, 2007; 282(35): 25903 - 25916. [Abstract] [Full Text] [PDF] |
||||
![]() |
K. Keyamura, N. Fujikawa, T. Ishida, S. Ozaki, M. Su'etsugu, K. Fujimitsu, W. Kagawa, S. Yokoyama, H. Kurumizaka, and T. Katayama The interaction of DiaA and DnaA regulates the replication cycle in E. coli by directly promoting ATP DnaA-specific initiation complexes Genes & Dev., August 15, 2007; 21(16): 2083 - 2099. [Abstract] [Full Text] [PDF] |
||||
![]() |
S. Jergic, K. Ozawa, N. K. Williams, X.-C. Su, D. D. Scott, S. M. Hamdan, J. A. Crowther, G. Otting, and N. E. Dixon The unstructured C-terminus of the {tau} subunit of Escherichia coli DNA polymerase III holoenzyme is the site of interaction with the {alpha} subunit Nucleic Acids Res., May 14, 2007; 35(9): 2813 - 2824. [Abstract] [Full Text] [PDF] |
||||
![]() |
D. E. Johnson, M. Takahashi, S. M. Hamdan, S.-J. Lee, and C. C. Richardson Exchange of DNA polymerases at the replication fork of bacteriophage T7 PNAS, March 27, 2007; 104(13): 5312 - 5317. [Abstract] [Full Text] [PDF] |
||||
![]() |
S. G. Anderson, C. R. Williams, M. O'Donnell, and L. B. Bloom A Function for the {psi} Subunit in Loading the Escherichia coli DNA Polymerase Sliding Clamp J. Biol. Chem., March 9, 2007; 282(10): 7035 - 7045. [Abstract] [Full Text] [PDF] |
||||
![]() |
E. V. Mirkin and S. M. Mirkin Replication Fork Stalling at Natural Impediments Microbiol. Mol. Biol. Rev., March 1, 2007; 71(1): 13 - 35. [Abstract] [Full Text] [PDF] |
||||
![]() |
T. W. Kirby, S. Harvey, E. F. DeRose, S. Chalov, A. K. Chikova, F. W. Perrino, R. M. Schaaper, R. E. London, and L. C. Pedersen Structure of the Escherichia coli DNA Polymerase III {epsilon}-HOT Proofreading Complex J. Biol. Chem., December 15, 2006; 281(50): 38466 - 38471. [Abstract] [Full Text] [PDF] |
||||
| ||||||||||||||||||||