Assembly of a chromosomal replication machine: two DNA polymerases, a clamp loader, and sliding clamps in one holoenzyme particle. IV. ATP-binding site mutants identify the clamp loader

The Mutant βE202K Sliding Clamp Protein Impairs DNA Polymerase III Replication Activity

Expression of the Escherichia coli dnaN-encoded β clamp at ≥10-fold higher than chromosomally expressed levels impedes growth by interfering with DNA replication. We hypothesized that the excess β clamp sequesters the replicative DNA polymerase III (Pol III) to inhibit replication. As a test of this hypothesis, we obtained eight mutant clamps with an inability to impede growth and measured their ability to stimulate Pol III replication in vitro. Compared with the wild-type clamp, seven of the mutants were defective, consistent with their elevated cellular levels failing to sequester Pol III. However, the β^E202K mutant that bears a glutamic acid-to-lysine substitution at residue 202 displayed an increased affinity for Pol IIIα and Pol III core (Pol IIIαεθ), suggesting that it could still sequester Pol III effectively. Of interest, β^E202K supported in vitro DNA replication by Pol II and Pol IV but was defective with Pol III. Genetic experiments indicated that the dnaN^E202K strain remained proficient in DNA damage-induced mutagenesis but was induced modestly for SOS and displayed sensitivity to UV light and methyl methanesulfonate. These results correlate an impaired ability of the mutant β^E202K clamp to support Pol III replication in vivo with its in vitro defect in DNA replication. Taken together, our results (i) support the model that sequestration of Pol III contributes to growth inhibition, (ii) argue for the existence of an additional mechanism that contributes to lethality, and (iii) suggest that physical and functional interactions of the β clamp with Pol III are more extensive than appreciated currently. IMPORTANCE The β clamp plays critically important roles in managing the actions of multiple proteins at the replication fork. However, we lack a molecular understanding of both how the clamp interacts with these different partners and the mechanisms by which it manages their respective actions. We previously exploited the finding that an elevated cellular level of the β clamp impedes Escherichia coli growth by interfering with DNA replication. Using a genetic selection method, we obtained novel mutant β clamps that fail to inhibit growth. Their analysis revealed that β^E202K is unique among them. Our work offers new insights into how the β clamp interacts with and manages the actions of E. coli DNA polymerases II, III, and IV.

Purification and interactions of the MucA' and MucB proteins constituting the DNA polymerase RI

Background

The MucA' and MucB proteins comprise the core of DNA polymerase RI which is a strong mutator utilized in mutagenicity assays such as the standard Ames test. A close relative DNA polymerase V, composed of the homologous UmuD' and UmuC proteins, is considered to be an ortholog of the mammalian DNA polymerase η. The catalytic subunits of these polymerases belong to the Y-family which specializes in the translesion DNA synthesis across various DNA adducts to rescue stalled chromosomal replication at the expense of mutations. Based on genetic evidence, DNA polymerase RI possesses the greatest ability to induce various types of mutations among all so far characterized members of the Y-superfamily. The exceptionally high mutagenic potential of MucA'B has been taken advantage of in numerous bacterial mutagenicity assays incorporating the conjugative plasmid pKM101 carrying the mucAB operon such as the Ames Test.

Results

We established new procedures for the purification of MucB protein as well as its accessory protein MucA' using the refolding techniques. The purified MucA' protein behaved as a molecular dimer which was fully stable in solution. The soluble monomeric form of MucB protein was obtained after refolding on a gel-filtration column and remained stable in a nondenaturing buffer containing protein aggregation inhibitors. Using the surface plasmon resonance technique, we demonstrated that the purified MucA' and MucB proteins interacted and that MucB protein preferentially bound to single-stranded DNA. In addition, we revealed that MucB protein interacted with the β-subunit of DNA polymerase III holoenzyme of E. coli.

Conclusion

The MucA' and MucB proteins can be isolated from inclusion bodies and solubilized in vitro. The refolded MucB protein interacts with its MucA' partner as well as with DNA what suggests it retains biological activity. The interaction of MucB with the processivity subunit of DNA polymerase III may imply the role of the subunit as an accessory protein to MucB during the translesion DNA synthesis.

Loading dynamics of a sliding DNA clamp

Sliding DNA clamps are loaded at a ss/dsDNA junction by a clamp loader that depends on ATP binding for clamp opening. Sequential ATP hydrolysis results in closure of the clamp so that it completely encircles and diffuses on dsDNA. We followed events during loading of an E. coli β clamp in real time by using single-molecule FRET (smFRET). Three successive FRET states were retained for 0.3 s, 0.7 s, and 9 min: Hydrolysis of the first ATP molecule by the γ clamp loader resulted in closure of the clamp in 0.3 s, and after 0.7 s in the closed conformation, the clamp was released to diffuse on the dsDNA for at least 9 min. An additional single-molecule polarization study revealed that the interfacial domain of the clamp rotated in plane by approximately 8° during clamp closure. The single-molecule polarization and FRET studies thus revealed the real-time dynamics of the ATP-hydrolysis-dependent 3D conformational change of the β clamp during loading at a ss/dsDNA junction.

Fate of the replisome following arrest by UV-induced DNA damage in Escherichia coli

Accurate replication in the presence of DNA damage is essential to genome stability and viability in all cells. In Escherichia coli, DNA replication forks blocked by UV-induced damage undergo a partial resection and RecF-catalyzed regression before synthesis resumes. These processing events generate distinct structural intermediates on the DNA that can be visualized in vivo using 2D agarose gels. However, the fate and behavior of the stalled replisome remains a central uncharacterized question. Here, we use thermosensitive mutants to show that the replisome's polymerases uncouple and transiently dissociate from the DNA in vivo. Inactivation of α, β, or τ subunits within the replisome is sufficient to signal and induce the RecF-mediated processing events observed following UV damage. By contrast, the helicase-primase complex (DnaB and DnaG) remains critically associated with the fork, leading to a loss of fork integrity, degradation, and aberrant intermediates when disrupted. The results reveal a dynamic replisome, capable of partial disassembly to allow access to the obstruction, while retaining subunits that maintain fork licensing and direct reassembly to the appropriate location after processing has occurred.

Replication clamps and clamp loaders

To achieve the high degree of processivity required for DNA replication, DNA polymerases associate with ring-shaped sliding clamps that encircle the template DNA and slide freely along it. The closed circular structure of sliding clamps necessitates an enzyme-catalyzed mechanism, which not only opens them for assembly and closes them around DNA, but specifically targets them to sites where DNA synthesis is initiated and orients them correctly for replication. Such a feat is performed by multisubunit complexes known as clamp loaders, which use ATP to open sliding clamp rings and place them around the 3' end of primer-template (PT) junctions. Here we discuss the structure and composition of sliding clamps and clamp loaders from the three domains of life as well as T4 bacteriophage, and provide our current understanding of the clamp-loading process.

Studying protein complexes by the yeast two-hybrid system

Protein complexes are typically analyzed by affinity purification and subsequent mass spectrometric analysis. However, in most cases the structure and topology of the complexes remains elusive from such studies. Here we investigate how the yeast two-hybrid system can be used to analyze direct interactions among proteins in a complex. First we tested all pairwise interactions among the seven proteins of Escherichia coli DNA polymerase III as well as an uncharacterized complex that includes MntR and PerR. Four and seven interactions were identified in these two complexes, respectively. In addition, we review Y2H data for three other complexes of known structure which serve as "gold-standards", namely Varicella Zoster Virus (VZV) ribonucleotide reductase (RNR), the yeast proteasome, and bacteriophage lambda. Finally, we review an Y2H analysis of the human spliceosome which may serve as an example for a dynamic mega-complex.

DNA replicases from a bacterial perspective

Bacterial replicases are complex, tripartite replicative machines. They contain a polymerase, polymerase III (Pol III), a β₂ processivity factor, and a DnaX complex ATPase that loads β₂ onto DNA and chaperones Pol III onto the newly loaded β₂. Bacterial replicases are highly processive, yet cycle rapidly during Okazaki fragment synthesis in a regulated way. Many bacteria encode both a full-length τ and a shorter γ form of DnaX by a variety of mechanisms. γ appears to be uniquely placed in a single position relative to two τ protomers in a pentameric ring. The polymerase catalytic subunit of Pol III, α, contains a PHP domain that not only binds to a prototypical ε Mg²⁺-dependent exonuclease, but also contains a second Zn²⁺-dependent proofreading exonuclease, at least in some bacteria. This review focuses on a critical evaluation of recent literature and concepts pertaining to the above issues and suggests specific areas that require further investigation.

A simplified method for reconstituting active E. coli DNA polymerase III

Genome duplication in E. coli is carried out by DNA polymerase III, an enzyme complex consisting of ten subunits. Investigations of the biochemical and structural properties of DNA polymerase III require the expression and purification of subunits including α, ge, θ, γ, δ', δ, and β separately followed by in vitro reconstitution of the pol III core and clamp loader. Here we propose a new method for expressing and purifying DNA polymerase III components by utilizing a protein co-expression strategy. Our results show that the subunits of the pol III core and those of the clamp loader can be coexpressed and purified based on inherent interactions between the subunits. The resulting pol III core, clamp loader and sliding clamp can be reconstituted effectively to perform DNA polymerization. Our strategy considerably simplifies the expression and purification of DNA polymerase III and provides a feasible and convenient method for exploring other multi-subunit systems.

The G157C mutation in the Escherichia coli sliding clamp specifically affects initiation of replication

Escherichia coli cells with a point mutation in the dnaN gene causing the amino acid change Gly157 to Cys, were found to underinitiate replication and grow with a reduced origin and DNA concentration. The mutant β clamp also caused excessive conversion of ATP-DnaA to ADP-DnaA. The DnaA protein was, however, not the element limiting initiation of replication. Overproduction of DnaA protein, which in wild-type cells leads to over-replication, had no effect in the dnaN(G157C) mutant. Origins already opened by DnaA seemed to remain open for a prolonged period, with a stage of initiation involving β clamp loading, presumably limiting the initiation process. The existence of opened origins led to a moderate SOS response. Lagging strand synthesis, which also requires loading of the β clamp, was apparently unaffected. The result indicates that some aspects of β clamp activity are specific to the origin. It is possible that the origin specific activities of β contribute to regulation of initiation frequency.

Coordinating DNA polymerase traffic during high and low fidelity synthesis

With the discovery that organisms possess multiple DNA polymerases (Pols) displaying different fidelities, processivities, and activities came the realization that mechanisms must exist to manage the actions of these diverse enzymes to prevent gratuitous mutations. Although many of the Pols encoded by most organisms are largely accurate, and participate in DNA replication and DNA repair, a sizeable fraction display a reduced fidelity, and act to catalyze potentially error-prone translesion DNA synthesis (TLS) past lesions that persist in the DNA. Striking the proper balance between use of these different enzymes during DNA replication, DNA repair, and TLS is essential for ensuring accurate duplication of the cell's genome. This review highlights mechanisms that organisms utilize to manage the actions of their different Pols. A particular emphasis is placed on discussion of current models for how different Pols switch places with each other at the replication fork during high fidelity replication and potentially error-pone TLS.

Conserved residues in the delta subunit help the E. coli clamp loader, gamma complex, target primer-template DNA for clamp assembly

The Escherichia coli clamp loader, γ complex (γ₃δδ′λψ), catalyzes ATP-driven assembly of β clamps onto primer-template DNA (p/tDNA), enabling processive replication. The mechanism by which γ complex targets p/tDNA for clamp assembly is not resolved. According to previous studies, charged/polar amino acids inside the clamp loader chamber interact with the double-stranded (ds) portion of p/tDNA. We find that dsDNA, not ssDNA, can trigger a burst of ATP hydrolysis by γ complex and clamp assembly, but only at far higher concentrations than p/tDNA. Thus, contact between γ complex and dsDNA is necessary and sufficient, but not optimal, for the reaction, and additional contacts with p/tDNA likely facilitate its selection as the optimal substrate for clamp assembly. We investigated whether a conserved sequence—HRVW₂₇₉QNRR—in δ subunit contributes to such interactions, since Tryptophan-279 specifically cross-links to the primer-template junction. Mutation of δ-W279 weakens γ complex binding to p/tDNA, hampering its ability to load clamps and promote proccessive DNA replication, and additional mutations in the sequence (δ-R277, δ-R283) worsen the interaction. These data reveal a novel location in the C-terminal domain of the E. coli clamp loader that contributes to DNA binding and helps define p/tDNA as the preferred substrate for the reaction.

Replisome mechanics: insights into a twin DNA polymerase machine

Chromosomal replicases are multicomponent machines that copy DNA with remarkable speed and processivity. The organization of the replisome reveals a twin DNA polymerase design ideally suited for concurrent synthesis of leading and lagging strands. Recent structural and biochemical studies of Escherichia coli and eukaryotic replication components provide intricate details of the organization and inner workings of cellular replicases. In particular, studies of sliding clamps and clamp-loader subunits elucidate the mechanisms of replisome processivity and lagging strand synthesis. These studies demonstrate close similarities between the bacterial and eukaryotic replication machineries.

Competition of bacteriophage polypeptides with native replicase proteins for binding to the DNA sliding clamp reveals a novel mechanism for DNA replication arrest in Staphylococcus aureus

Bacteriophages have evolved specific mechanisms that redirect bacterial metabolic pathways to the bacteriophage reproduction cycle. In this study, we characterized the bactericidal mechanism of two polypeptides from bacteriophages Twort and G1 that target the DNA sliding clamp of Staphylococcus aureus. The DNA sliding clamp, which tethers DNA polymerase to its template and thereby confers processivity upon the enzyme, was found to be essential for the viability of S. aureus. Expression of polypeptides TwortORF168 and G1ORF240 in S. aureus selectively inhibited DNA replication which in turn resulted in cell death. Both polypeptides specifically inhibited the S. aureus DNA replicase that was reconstituted in vitro but not the corresponding replicase of Streptococcus pyogenes. We demonstrated that inhibition of DNA synthesis is multifaceted and occurs via binding the DNA sliding clamp: TwortORF168 and G1ORF240 bound tightly to the DNA sliding clamp and prevented both its loading onto DNA and its interaction with DNA polymerase C. These results elucidate the impact of bacteriophage polypeptide expression upon DNA replication in the growing cell.

Dynamics of loading the Escherichia coli DNA polymerase processivity clamp

Sliding clamps and clamp loaders are processivity factors required for efficient DNA replication. Sliding clamps are ring-shaped complexes that tether DNA polymerases to DNA to increase the processivity of synthesis. Clamp loaders assemble these ring-shaped clamps onto DNA in an ATP-dependent reaction. The overall process of clamp loading is dynamic in that protein-protein and protein-DNA interactions must actively change in a coordinated fashion to complete the mechanical clamp-loading reaction cycle. The clamp loader must initially have a high affinity for both the clamp and DNA to bring these macromolecules together, but then must release the clamp on DNA for synthesis to begin. Evidence is presented for a mechanism in which the clamp-loading reaction comprises a series of binding reactions to ATP, the clamp, DNA, and ADP, each of which promotes some change in the conformation of the clamp loader that alters interactions with the next component of the pathway. These changes in interactions must be rapid enough to allow the clamp loader to keep pace with replication fork movement. This review focuses on the measurement of dynamic and transient interactions required to assemble the Escherichia coli sliding clamp on DNA.

Cellular DNA replicases: components and dynamics at the replication fork

DNA replicases are multicomponent machines that have evolved clever strategies to perform their function. Although the structure of DNA is elegant in its simplicity, the job of duplicating it is far from simple. At the heart of the replicase machinery is a heteropentameric AAA+ clamp-loading machine that couples ATP hydrolysis to load circular clamp proteins onto DNA. The clamps encircle DNA and hold polymerases to the template for processive action. Clamp-loader and sliding clamp structures have been solved in both prokaryotic and eukaryotic systems. The heteropentameric clamp loaders are circular oligomers, reflecting the circular shape of their respective clamp substrates. Clamps and clamp loaders also function in other DNA metabolic processes, including repair, checkpoint mechanisms, and cell cycle progression. Twin polymerases and clamps coordinate their actions with a clamp loader and yet other proteins to form a replisome machine that advances the replication fork.

Fluorescence measurements on the E.coli DNA polymerase clamp loader: implications for conformational changes during ATP and clamp binding

Sliding clamps are ring-shaped proteins that tether DNA polymerases to their templates during processive DNA replication. The action of ATP-dependent clamp loader complexes is required to open the circular clamps and to load them onto DNA. The crystal structure of the pentameric clamp loader complex from Escherichia coli (the gamma complex), determined in the absence of nucleotides, revealed a highly asymmetric and extended form of the clamp loader. Consideration of this structure suggested that a compact and more symmetrical inactive form may predominate in solution in the absence of crystal packing forces. This model has the N-terminal domains of the delta and delta' subunits of the clamp loader close to each other in the inactive state, with the clamp loader opening in a crab-claw-like fashion upon ATP-binding. We have used fluorescence resonance energy transfer (FRET) to investigate the structural changes in the E.coli clamp loader complex that result from ATP-binding and interactions between the clamp loader and the beta clamp. FRET measurements using fluorophores placed in the N-terminal domains of the delta and delta' subunits indicate that the distances between these subunits in solution are consistent with the previously crystallized extended form of the clamp loader. Furthermore, the addition of nucleotide and clamp to the labeled clamp loader does not appreciably alter these FRET distances. Our results suggest that the changes that occur in the relative positioning of the delta and delta' subunits when ATP binds to and activates the complex are subtle, and that crab-claw-like movements are not a significant component of the clamp loader mechanism.

Distinct roles for ATP binding and hydrolysis at individual subunits of an archaeal clamp loader

Circular clamps are utilised by replicative polymerases to enhance processivity. The topological problem of loading a toroidal clamp onto DNA is overcome by ATP-dependent clamp loader complexes. Different organisms use related protein machines to load clamps, but the mechanisms by which they utilise ATP are surprisingly different. Using mutant clamp loaders that are deficient in either ATP binding or hydrolysis in different subunits, we show how the different subunits of an archaeal clamp loader use ATP binding and hydrolysis in distinct ways at different steps in the loading process. Binding of nucleotide by the large subunit and three of the four small subunits is sufficient for clamp loading. However, ATP hydrolysis by the small subunits is required for release of PCNA to allow formation of the complex between PCNA and the polymerase, while hydrolysis by the large subunit is required for catalytic clamp loading.

The PCNA-RFC families of DNA clamps and clamp loaders

The proliferating cell nuclear antigen PCNA functions at multiple levels in directing DNA metabolic pathways. Unbound to DNA, PCNA promotes localization of replication factors with a consensus PCNA-binding domain to replication factories. When bound to DNA, PCNA organizes various proteins involved in DNA replication, DNA repair, DNA modification, and chromatin modeling. Its modification by ubiquitin directs the cellular response to DNA damage. The ring-like PCNA homotrimer encircles double-stranded DNA and slides spontaneously across it. Loading of PCNA onto DNA at template-primer junctions is performed in an ATP-dependent process by replication factor C (RFC), a heteropentameric AAA+ protein complex consisting of the Rfc1, Rfc2, Rfc3, Rfc4, and Rfc5 subunits. Loading of yeast PCNA (POL30) is mechanistically distinct from analogous processes in E. coli (beta subunit by the gamma complex) and bacteriophage T4 (gp45 by gp44/62). Multiple stepwise ATP-binding events to RFC are required to load PCNA onto primed DNA. This stepwise mechanism should permit editing of this process at individual steps and allow for divergence of the default process into more specialized modes. Indeed, alternative RFC complexes consisting of the small RFC subunits together with an alternative Rfc1-like subunit have been identified. A complex required for the DNA damage checkpoint contains the Rad24 subunit, a complex required for sister chromatid cohesion contains the Ctf18 subunit, and a complex that aids in genome stability contains the Elg1 subunit. Only the RFC-Rad24 complex has a known associated clamp, a heterotrimeric complex consisting of Rad17, Mec3, and Ddc1. The other putative clamp loaders could either act on clamps yet to be identified or act on the two known clamps.

Suppression of temperature-sensitive chromosome replication of an Escherichia coli dnaX(Ts) mutant by reduction of initiation efficiency

Temperature sensitivity of DNA polymerization and growth of a dnaX(Ts) mutant is suppressible at 39 to 40 degrees C by mutations in the initiator gene, dnaA. These suppressor mutations concomitantly cause initiation inhibition at 20 degrees C and have been designated Cs,Sx to indicate both phenotypic characteristics of cold-sensitive initiation and suppression of dnaX(Ts). One dnaA(Cs,Sx) mutant, A213D, has reduced affinity for ATP, and two mutants, R432L and T435K, have eliminated detectable DnaA box binding in vitro. Two models have explained dnaA(Cs,Sx) suppression of dnaX, which codes for both the tau and gamma subunits of DNA polymerase III. The initiation deficiency model assumes that reducing initiation efficiency allows survival of the dnaX(Ts) mutant at the somewhat intermediate temperature of 39 to 40 degrees C by reducing chromosome content per cell, thus allowing partially active DNA polymerase III to complete replication of enough chromosomes for the organism to survive. The stabilization model is based on the idea that DnaA interacts, directly or indirectly, with polymerization factors during replication. We present five lines of evidence consistent with the initiation deficiency model. First, a dnaA(Cs,Sx) mutation reduced initiation frequency and chromosome content (measured by flow cytometry) and origin/terminus ratios (measured by real-time PCR) in both wild-type and dnaX(Ts) strains growing at 39 and 34 degrees C. These effects were shown to result specifically from the Cs,Sx mutations, because the dnaX(Ts) mutant is not defective in initiation. Second, reduction of the number of origins and chromosome content per cell was common to all three known suppressor mutations. Third, growing the dnaA(Cs,Sx) dnaX(Ts) strain on glycerol-containing medium reduced its chromosome content to one per cell and eliminated suppression at 39 degrees C, as would be expected if the combination of poor carbon source, the Cs,Sx mutation, the Ts mutation, and the 39 degrees C incubation reduced replication to the point that growth (and, therefore, suppression) was not possible. However, suppression was possible on glycerol medium at 38 degrees C. Fourth, the dnaX(Ts) mutation can be suppressed also by introduction of oriC mutations, which reduced initiation efficiency and chromosome number per cell, and the degree of suppression was proportional to the level of initiation defect. Fifth, introducing a dnaA(Cos) allele, which causes overinitiation, into the dnaX(Ts) mutant exacerbated its temperature sensitivity.

Motors and switches: AAA+ machines within the replisome

Clamp loaders are required to load the ring-shaped clamps that tether replicative DNA polymerases onto DNA. Recently solved crystal structures, along with a series of biochemical studies, have provided a detailed understanding of the clamp loading reaction. In particular, studies of the Escherichia coli clamp loader--an AAA+ machine--have provided insights into the architecture of clamp loaders from eukaryotes, bacteriophage T4 and archaea. Other AAA+ proteins are also involved in the initiation of DNA replication, and studies of the E. coli clamp loader indicate mechanisms by which these proteins might function.

Clamp loaders and sliding clamps

A coherent view of the structure and function of DNA polymerase processivity factors (sliding clamps and clamp loaders) is emerging from recent structural studies. Crystal structures of sliding clamps from the T4 and RB69 bacteriophages, and from an archaebacterium expand the gallery of ring-shaped processivity factors and clarify how the clamp interacts with the DNA polymerase. Crystallographic and electron microscopic views of clamp loaders from bacteria, archaebacteria and eukaryotes emphasize their common architecture and have produced models of how ATPbinding might be coupled to clamp opening/loading.

Replisome-mediated DNA replication

The elaborate process of genomic replication requires a large collection of proteins properly assembled at a DNA replication fork. Several decades of research on the bacterium Escherichia coli and its bacteriophages T4 and T7 have defined the roles of many proteins central to DNA replication. These three different prokaryotic replication systems use the same fundamental components for synthesis at a moving DNA replication fork even though the number and nature of some individual proteins are different and many lack extensive sequence homology. The components of the replication complex can be grouped into functional categories as follows: DNA polymerase, helix destabilizing protein, polymerase accessory factors, and primosome (DNA helicase and DNA primase activities). The replication of DNA derives from a multistep enzymatic pathway that features the assembly of accessory factors and polymerases into a functional holoenzyme; the separation of the double-stranded template DNA by helicase activity and its coupling to the primase synthesis of RNA primers to initiate Okazaki fragment synthesis; and the continuous and discontinuous synthesis of the leading and lagging daughter strands by the polymerases. This review summarizes and compares and contrasts for these three systems the types, timing, and mechanism of reactions and of protein-protein interactions required to initiate, control, and coordinate the synthesis of the leading and lagging strands at a DNA replication fork and comments on their generality.

ATP utilization by yeast replication factor C. III. The ATP-binding domains of Rfc2, Rfc3, and Rfc4 are essential for DNA recognition and clamp loading

The conserved lysine in the Walker A motif of the ATP-binding domain encoded by the yeast RFC1, RFC2, RFC3, and RFC4 genes was mutated to glutamic acid. Complexes of replication factor C with a N-terminal truncation (Delta2-273) of the Rfc1 subunit (RFC) containing a single mutant subunit were overproduced in Escherichia coli for biochemical analysis. All of the mutant RFC complexes were capable of interacting with PCNA. Complexes containing a rfc1-K359E mutation were similar to wild type in replication activity and ATPase activity; however, the mutant complex showed increased susceptibility to proteolysis. In contrast, complexes containing either a rfc2-K71E mutation or a rfc3-K59E mutation were severely impaired in ATPase and clamp loading activity. In addition to their defects in ATP hydrolysis, these complexes were defective for DNA binding. A mutant complex containing the rfc4-K55E mutation performed as well as a wild type complex in clamp loading, but only at very high ATP concentrations. Mutant RFC complexes containing rfc2-K71R or rfc3-K59R, carrying a conservative lysine --> arginine mutation, had much milder clamp loading defects that could be partially (rfc2-K71R) or completely (rfc3-K59R) suppressed at high ATP concentrations.

The delta subunit of DNA polymerase III holoenzyme serves as a sliding clamp unloader in Escherichia coli

In Escherichia coli, the circular beta sliding clamp facilitates processive DNA replication by tethering the polymerase to primer-template DNA. When synthesis is complete, polymerase dissociates from beta and DNA and cycles to a new start site, a primed template loaded with beta. DNA polymerase cycles frequently during lagging strand replication while synthesizing 1-2-kilobase Okazaki fragments. The clamps left behind remain stable on DNA (t(12) approximately 115 min) and must be removed rapidly for reuse at numerous primed sites on the lagging strand. Here we show that delta, a single subunit of DNA polymerase III holoenzyme, opens beta and slips it off DNA (k(unloading) = 0.011 s(-)(1)) at a rate similar to that of the multisubunit gamma complex clamp loader by itself (0.015 s(-)(1)) or within polymerase (pol) III* (0.0065 s(-)(1)). Moreover, unlike gamma complex and pol III*, delta does not require ATP to catalyze clamp unloading. Quantitation of gamma complex subunits (gamma, delta, delta', chi, psi) in E. coli cells reveals an excess of delta, free from gamma complex and pol III*. Since pol III* and gamma complex occur in much lower quantities and perform several DNA metabolic functions in replication and repair, the delta subunit probably aids beta clamp recycling during DNA replication.

Escherichia coli DNA polymerase III tau- and gamma-subunit conserved residues required for activity in vivo and in vitro

The Escherichia coli DNA polymerase III tau and gamma subunits are single-strand DNA-dependent ATPases (the latter requires the delta and delta' subunits for significant ATPase activity) involved in loading processivity clamp beta. They are homologous to clamp-loading proteins of many organisms from phages to humans. Alignment of 27 prokaryotic tau/gamma homologs and 1 eukaryotic tau/gamma homolog has refined the sequences of nine previously defined identity and functional motifs. Mutational analysis has defined highly conserved residues required for activity in vivo and in vitro. Specifically, mutations introduced into highly conserved residues within three of those motifs, the P loop, the DExx region, and the SRC region, inactivated complementing activity in vivo and clamp loading in vitro and reduced ATPase catalytic efficiency in vitro. Mutation of a highly conserved residue within a fourth motif, VIc, inactivated clamp-loading activity and reduced ATPase activity in vitro, but the mutant gene, on a multicopy plasmid, retained complementing activity in vivo and the mutant gene also supported apparently normal replication and growth as a haploid, chromosomal allele.

DNA structure requirements for the Escherichia coli gamma complex clamp loader and DNA polymerase III holoenzyme

The Escherichia coli chromosomal replicase, DNA polymerase III holoenzyme, is highly processive during DNA synthesis. Underlying high processivity is a ring-shaped protein, the beta clamp, that encircles DNA and slides along it, thereby tethering the enzyme to the template. The beta clamp is assembled onto DNA by the multiprotein gamma complex clamp loader that opens and closes the beta ring around DNA in an ATP-dependent manner. This study examines the DNA structure required for clamp loading action. We found that the gamma complex assembles beta onto supercoiled DNA (replicative form I), but only at very low ionic strength, where regions of unwound DNA may exist in the duplex. Consistent with this, the gamma complex does not assemble beta onto relaxed closed circular DNA even at low ionic strength. Hence, a 3'-end is not required for clamp loading, but a single-stranded DNA (ssDNA)/double-stranded DNA (dsDNA) junction can be utilized as a substrate, a result confirmed using synthetic oligonucleotides that form forked ssDNA/dsDNA junctions on M13 ssDNA. On a flush primed template, the gamma complex exhibits polarity; it acts specifically at the 3'-ssDNA/dsDNA junction to assemble beta onto the DNA. The gamma complex can assemble beta onto a primed site as short as 10 nucleotides, corresponding to the width of the beta ring. However, a protein block placed closer than 14 base pairs (bp) upstream from the primer 3' terminus prevents the clamp loading reaction, indicating that the gamma complex and its associated beta clamp interact with approximately 14-16 bp at a ssDNA/dsDNA junction during the clamp loading operation. A protein block positioned closer than 20-22 bp from the 3' terminus prevents use of the clamp by the polymerase in chain elongation, indicating that the polymerase has an even greater spatial requirement than the gamma complex on the duplex portion of the primed site for function with beta. Interestingly, DNA secondary structure elements placed near the 3' terminus impose similar steric limits on the gamma complex and polymerase action with beta. The possible biological significance of these structural constraints is discussed.

DNA polymerase of the T4-related bacteriophages

The DNA polymerase of bacteriophage T4, product of phage gene 43 (gp43), has served as a model replicative DNA polymerase in nucleic acids research for nearly 40 years. The base-selection (polymerase, or Pol) and editing (3'-exonuclease, or Exo) functions of this multifunctional protein, which have counterparts in the replicative polymerases of other organisms, are primary determinants of the high fidelity of DNA synthesis in phage DNA replication. T4 gp43 is considered to be a member of the "B family" of DNA-dependent DNA polymerases (those resembling eukaryotic Pol alpha) because it exhibits striking similarities in primary structure to these enzymes. It has been extensively analyzed at the genetic, physiological, and biochemical levels; however, relationships between the in vivo properties of this enzyme and its physical structure have not always been easy to explain due to a paucity of structural data on the intact molecule. However, gp43 from phage RB69, a phylogenetic relative of T4, was crystallized and its structure solved in a complex with single-stranded DNA occupying the Exo site, as well as in the unliganded form. Analyses with these crystals, and crystals of a T4 gp43 proteolytic fragment harboring the Exo function, are opening new avenues to interpret existing biological and biochemical data on the intact T4 enzyme and are revealing new aspects of the microanatomy of gp43 that can now be explored further for functional significance. We summarize our current understanding of gp43 structure and review the physiological roles of this protein as an essential DNA-binding component of the multiprotein T4 DNA replication complex and as a nucleotide-sequence-specific RNA-binding translational repressor that controls its own biosynthesis and activity in vivo. We also contrast the properties of the T4 DNA replication complex to the functionally analogous complexes of other organisms, particularly Escherichia coli, and point out some of the unanswered questions about gp43 and T4 DNA replication.

The DnaX-binding subunits delta' and psi are bound to gamma and not tau in the DNA polymerase III holoenzyme

The DnaX complex subassembly of the DNA polymerase III holoenzyme is comprised of the DnaX proteins tau and gamma and the auxiliary subunits delta, delta', chi, and psi, which together load the beta processivity factor onto primed DNA in an ATP-dependent reaction. delta' and psi bind directly to DnaX whereas delta and chi bind to delta' and psi, respectively (Onrust, R., Finkelstein, J., Naktinis, V., Turner, J., Fang, L., and O'Donnell, M. (1995) J. Biol. Chem. 270, 13348-13357). Until now, it has been unclear which DnaX protein, tau or gamma, in holoenzyme binds the auxiliary subunits delta, delta', chi,and psi. Treatment of purified holoenzyme with the homobifunctional cross-linker bis(sulfosuccinimidyl)suberate produces covalently cross-linked gamma-delta' and gamma-psi complexes identified by Western blot analysis. Immunodetection of cross-linked species with anti-delta' and anti-psi antibodies revealed that no tau-delta' or tau-psi cross-links had formed, suggesting that the delta' and psi subunits reside only on gamma within holoenzyme.

A model for Escherichia coli DNA polymerase III holoenzyme assembly at primer/template ends. DNA triggers a change in binding specificity of the gamma complex clamp loader

The gamma complex of the Escherichia coli DNA polymerase III holoenzyme assembles the beta sliding clamp onto DNA in an ATP hydrolysis-driven reaction. Interactions between gamma complex and primer/template DNA are investigated using fluorescence depolarization to measure binding of gamma complex to different DNA substrates under steady-state and presteady-state conditions. Surprisingly, gamma complex has a much higher affinity for single-stranded DNA (K(d) in the nM range) than for a primed template (K(d) in the microM range) under steady-state conditions. However, when examined on a millisecond time scale, we find that gamma complex initially binds very rapidly and with high affinity to primer/template DNA but is converted subsequently to a much lower affinity DNA binding state. Presteady-state data reveals an effective dissociation constant of 1.5 nM for the initial binding of gamma complex to DNA and a dissociation constant of 5.7 microM for the low affinity DNA binding state. Experiments using nonhydrolyzable ATPgammaS show that ATP binding converts gamma complex from a low affinity "inactive" to high affinity "active" DNA binding state while ATP hydrolysis has the reverse effect, thus allowing cycling between active and inactive DNA binding forms at steady-state. We propose that a DNA-triggered switch between active and inactive states of gamma complex provides a two-tiered mechanism enabling gamma complex to recognize primed template sites and load beta, while preventing gamma complex from competing with DNA polymerase III core for binding a newly loaded beta.DNA complex.

The internal workings of a DNA polymerase clamp-loading machine

Replicative DNA polymerases are multiprotein machines that are tethered to DNA during chain extension by sliding clamp proteins. The clamps are designed to encircle DNA completely, and they are manipulated rapidly onto DNA by the ATP-dependent activity of a clamp loader. We outline the detailed mechanism of gamma complex, a five-protein clamp loader that is part of the Escherichia coli replicase, DNA polymerase III holoenzyme. The gamma complex uses ATP to open the beta clamp and assemble it onto DNA. Surprisingly, ATP is not needed for gamma complex to crack open the beta clamp. The function of ATP is to regulate the activity of one subunit, delta, which opens the clamp simply by binding to it. The delta' subunit acts as a modulator of the interaction between delta and beta. On binding ATP, the gamma complex is activated such that the delta' subunit permits delta to bind beta and crack open the ring at one interface. The clamp loader-open clamp protein complex is now ready for an encounter with primed DNA to complete assembly of the clamp around DNA. Interaction with DNA stimulates ATP hydrolysis which ejects the gamma complex from DNA, leaving the ring to close around the duplex.

ATP hydrolysis catalyzed by human replication factor C requires participation of multiple subunits

Human replication factor C (hRFC) is a five-subunit protein complex (p140, p40, p38, p37, and p36) that acts to catalytically load proliferating cell nuclear antigen onto DNA, where it recruits DNA polymerase delta or epsilon to the primer terminus at the expense of ATP, leading to processive DNA synthesis. We have previously shown that a subcomplex of hRFC consisting of three subunits (p40, p37, and p36) contained DNA-dependent ATPase activity. However, it is not clear which subunit(s) hydrolyzes ATP, as all five subunits include potential ATP binding sites. In this report, we introduced point mutations in the putative ATP-binding sequences of each hRFC subunit and examined the properties of the resulting mutant hRFC complex and the ATPase activity of the hRFC or the p40.p37.p36 complex. A mutation in any one of the ATP binding sites of the p36, p37, p40, or p140 subunits markedly reduced replication activity of the hRFC complex and the ATPase activity of the hRFC or the p40.p37.p36 complex. A mutation in the ATP binding site of the p38 subunit did not alter the replication activity of hRFC. These findings indicate that the replication activity of hRFC is dependent on efficient ATP hydrolysis contributed to by the action of four hRFC subunits.

ATP binding to the Escherichia coli clamp loader powers opening of the ring-shaped clamp of DNA polymerase III holoenzyme

The Escherichia coli gamma complex serves as a clamp loader, catalyzing ATP-dependent assembly of beta protein clamps onto primed DNA templates during DNA replication. These ring-shaped clamps tether DNA polymerase III holoenzyme to the template, facilitating rapid and processive DNA synthesis. This report focuses on the role of ATP binding and hydrolysis catalyzed by the gamma complex during clamp loading. We show that the energy from ATP binding to gamma complex powers several initial events in the clamp loading pathway. The gamma complex (gamma2 delta delta'chi psi) binds two ATP molecules (one per gamma subunit in the complex) with high affinity (Kd = 1-2. 5 x 10(-6) M) or two adenosine 5'-O-(3-thiotriphosphate)(ATPgammaS) molecules with slightly lower affinity (Kd = 5-6.5 x 10(-6) M). Experiments performed prior to the first ATP turnover (kcat = 4 x 10(-3) s-1 at 4 degreesC), or in the presence of ATPgammaS (kcat = 1 x 10(-4) s-1 at 37 degreesC), demonstrate that upon interaction with ATP the gamma complex undergoes a change in conformation. This ATP-bound gamma complex binds beta and opens the ring at the dimer interface. Still prior to ATP hydrolysis, the composite of gamma complex and the open beta ring binds with high affinity to primer-template DNA. Thus ATP binding powers all the steps in the clamp loading pathway leading up to the assembly of a gamma complex. open beta ring.DNA intermediate, setting the stage for ring closing and turnover of the clamp loader, steps that may be linked to subsequent hydrolysis of ATP.

Reconstitution of recombinant human replication factor C (RFC) and identification of an RFC subcomplex possessing DNA-dependent ATPase activity

Replication factor C (RFC) is a five-subunit protein complex required for coordinate leading and lagging strand DNA synthesis during S phase and DNA repair in eukaryotic cells. It functions to load the proliferating cell nuclear antigen (PCNA), a processivity factor for polymerases delta and epsilon, onto primed DNA templates. This process, which is ATP-dependent, is carried out by 1) recognition of the primer terminus by RFC () binding to and disruption of the PCNA trimer, and then 3) topologically linking the PCNA to the DNA. In this report, we describe the purification and properties of recombinant human RFC expressed in Sf9 cells from baculovirus expression vectors. Like native RFC derived from 293 cells, recombinant RFC was found to support SV40 DNA synthesis and polymerase delta DNA synthesis in vitro and to possess an ATPase activity that was highly stimulated by DNA and further augmented by PCNA. Assembly of RFC was observed to involve distinct subunit interactions in which both the 36- and 38-kDa subunits interacted with the 37-kDa subunit, and the 40-kDa subunit interacted with the 36-kDa subunit-37-kDa subunit subcomplex. The 140-kDa subunit was found to require interactions primarily with the 38- and 40-kDa subunits for incorporation into the complex. In addition, a stable subcomplex lacking the 140-kDa subunit, although defective for DNA replication, was found to possess DNA-dependent ATPase activity that was not responsive to the addition of PCNA.

Assembly and disassembly of DNA polymerase holoenzyme

The complex task of genomic replication requires a large collection of proteins properly assembled within the close confines of the replication fork. The mechanism and dynamics of holoenzyme assembly and disassembly have been investigated using steady state and pre-steady state methods as opposed to structural studies, primarily due to the intrinsic transient nature of these protein complexes during DNA replication. The key step in bacteriophage T4 holoenzyme assembly involves ATP hydrolysis, whereas disassembly is mediated by subunit dissociation of the clamp protein in an ATP-independent manner.

A complex consisting of human replication factor C p40, p37, and p36 subunits is a DNA-dependent ATPase and an intermediate in the assembly of the holoenzyme

Human replication factor C (hRFC) is a multi-subunit protein complex capable of supporting proliferating cell nuclear antigen (PCNA)-dependent DNA synthesis by DNA polymerases delta and epsilon. The hRFC complex consists of five different subunits with apparent molecular masses of 140, 40, 38, 37, and 36 kDa. We have previously reported the expression of a three-subunit core complex, consisting of the p40, p37, and p36 subunits following coupled in vitro transcription-translation of the cDNAs encoding these proteins (Uhlmann, F., Cai, J., Flores-Rozas, H., Dean, F. B., Finkelstein, J. , O'Donnell, M., and Hurwitz, J. (1996) Proc. Natl. Acad. Sci. U. S. A. 93, 6521-6526). Here we describe the isolation of a stable complex composed of the p40, p37, and p36 subunits of hRFC from baculovirus-infected insect cells. The purified p40.p37.p36 complex, like the five-subunit RFC, contained DNA-dependent ATPase activity that was stimulated by PCNA, preferentially bound to primed DNA templates, interacted with PCNA, and was capable of unloading PCNA from singly-nicked circular DNA. In contrast to the five-subunit RFC, the three-subunit core complex did not load PCNA onto DNA. The p40. p37.p36 complex inhibited the elongation of primed DNA templates catalyzed by the DNA polymerase delta holoenzyme. Incubation of the p40.p37.p36 complex with the hRFC p140 and p38 subunits formed the five-subunit hRFC complex that supported PCNA-dependent DNA synthesis by DNA polymerase delta.

Conservation of the Escherichia coli dnaX programmed ribosomal frameshift signal in Salmonella typhimurium

Escherichia coli DNA polymerase III subunits tau and gamma are produced from one gene, dnaX, by a programmed ribosomal frameshift which generates the C terminal of gamma within the tau reading frame. To help evaluate the role of the dispensable gamma, the distribution of tau and gamma homologs in several other species and the sequence of the Salmonella typhimurium dnaX were determined. All four enterobacteria tested produce tau and gamma homologs. S. typhimurium dnaX is 83% identical to E. coli dnaX, but all four components of the frameshift signal are 100% conserved.

Protein-protein and protein-DNA interactions at the bacteriophage T4 DNA replication fork. Characterization of a fluorescently labeled DNA polymerase sliding clamp

The T4 DNA polymerase holoenzyme is composed of the polymerase enzyme complexed to the sliding clamp (the 45 protein), which is loaded onto DNA by an ATP-dependent clamp loader (the 44/62 complex). This paper describes a new method to directly investigate the mechanism of holoenzyme assembly using a fluorescently labeled cysteine mutant of the 45 protein. This protein possessed unaltered function yet produced substantial changes in probe fluorescence intensity upon interacting with other components of the holoenzyme. These fluorescence changes provide insight into the role of ATP hydrolysis in holoenzyme assembly. Using either ATP or the non-hydrolyzable ATP analog, adenosine 5'-O-(3-thiophosphate), events in holoenzyme assembly were assigned as either dependent or independent of ATP hydrolysis. A holoenzyme assembly mechanism is proposed in which the 44/62 complex mediates the association of the 45 protein with DNA in an ATP-dependent manner not requiring ATP hydrolysis. Upon ATP hydrolysis, the 44/62 complex triggers a conformational change in the 45 protein that may be attributed to the clamp loading onto DNA.

In vivo assembly of the tau-complex of the DNA polymerase III holoenzyme expressed from a five-gene artificial operon. Cleavage of the tau-complex to form a mixed gamma-tau-complex by the OmpT protease

A plasmid was constructed that encodes all five subunits of the Escherichia coli tau-complex on a single artificially constructed operon under the control of an inducible promoter. The proteins tau, delta, delta , chi, and psi overproduced from this artificial operon assemble efficiently in vivo, providing an efficient source of homogeneous tau-complex. The gamma subunit is a truncated form of tau that is produced by a translational frameshift. When protein expression was induced in bacterial strains containing the outer membrane protein T (OmpT) protease, tau was proteolyzed after lysis to a gamma-like protein, gammaP, and a peptide, C-tau, corresponding to the C terminus of tau. N-terminal sequencing of C-tau revealed a cleavage site between two lysines at positions 429 and 430 of tau. The deduced sequence of gammaP is, therefore, only two amino acids shorter than natural gamma. The proteolysis by OmpT was also shown directly by using purified OmpT and tau-complex in an in vitro reaction. A gammaP-complex and a mixed tau-gammaP-complex were purified from ompT+ cells. When the tau-complex proteins were overexpressed in ompT- bacteria, intact tau-complex lacking gammaP could be purified.

Assembly of a chromosomal replication machine: two DNA polymerases, a clamp loader, and sliding clamps in one holoenzyme particle. I. Organization of the clamp loader

The gamma complex of DNA polymerase III holoenzyme, the replicase of Escherichia coli, couples ATP hydrolysis to the loading of beta sliding clamps onto primed DNA. The beta sliding clamp tethers the holoenzyme replicase to DNA for rapid and processive synthesis. In this report, the gamma complex has been constituted from its five different subunits. Size measurements and subunit stoichiometry studies show a composition of gamma 2 delta 1 delta' 1 1 chi 1 psi 1. Strong intersubunit contacts have been identified by gel filtration, and weaker contacts were identified by surface plasmon resonance measurements. An analogous tau complex has also been constituted and characterized; it is nearly as active as the gamma complex in clamp loading activity, but as shown in the fourth report of this series, it is at a disadvantage in binding the delta, delta', chi, and psi subunits when core is present (Xiao, H., Naktinis, V., and O'Donnell, M. (1995) J. Biol. Chem. 270, 13378-13383). The single copy subunits within the gamma complex provide the basis for the structural asymmetry inherent within DNA polymerase III holoenzyme.

Assembly of a chromosomal replication machine: two DNA polymerases, a clamp loader, and sliding clamps in one holoenzyme particle. III. Interface between two polymerases and the clamp loader

The nine-subunit DNA polymerase (Pol) III* coupled to its beta sliding clamp is a rapid and highly processive replicating machine. The multiple subunits are needed for the complicated task of duplicating the Escherichia coli chromosome. In this report, Pol III* was constituted from individual pure proteins, and its structure was studied. Constitution of the Pol III* particle requires an ordered addition of the subunits, and the final structure contains 14 polypeptides in the ratio alpha 2 epsilon 2 theta 2 tau 2 gamma 2 delta 1 delta' 1 chi 1 psi 1. The structure can be summarized as being composed of two core polymerases (alpha epsilon theta) held together by a dimer of tau and one gamma complex clamp loader (gamma 2 delta 1 delta' 1 chi 1 psi 1) for loading beta onto DNA. At the center of the structure, the related tau and gamma subunits form a heterotetramer upon which the two core polymerases and clamp loader proteins assemble. The single copy nature of the delta, delta', chi, and psi subunits confers a structural asymmetry with respect to the two polymerases, presumably for the different functions of replicating the leading and lagging strands.