A possible mechanism for the dynamics of transition between polymerase and exonuclease sites in a high-fidelity DNA polymerase

The fidelity of DNA synthesis by DNA polymerase is significantly increased by a mechanism of proofreading that is performed at the exonuclease active site separate from the polymerase active site. Thus, the transition of DNA between the two active sites is an important activity of DNA polymerase. Here, based on our proposed model, the rates of DNA transition between the two active sites are theoretically studied. With the relevant parameters, which are determined from the available crystal structure and other experimental data, the calculated transfer rate of correctly base-paired DNA from the polymerase to exonuclease sites and the transfer rate after incorporation of a mismatched base are in good agreement with the available experimental data. The transfer rates in the presence of two and three mismatched bases are also consistent with the previous experimental data. In addition, the calculated transfer rate from the exonuclease to polymerase sites has a large value even with the high binding affinity of 3′-5′ ssDNA for the exonuclease site, which is also consistent with the available experimental value. Moreover, we also give some predictive results for the transfer rate of DNA containing only A:T base pairs and that of DNA containing only G:C base pairs.     

Proofreading dynamics of a processive DNA polymerase

Replicative DNA polymerases present an intrinsic proofreading activity during which the DNA primer chain is transferred between the polymerization and exonuclease sites of the protein. The dynamics of this primer transfer reaction during active polymerization remain poorly understood. Here we describe a single‐molecule mechanical method to investigate the conformational dynamics of the intramolecular DNA primer transfer during the processive replicative activity of the Φ29 DNA polymerase and two of its mutants. We find that mechanical tension applied to a single polymerase–DNA complex promotes the intramolecular transfer of the primer in a similar way to the incorporation of a mismatched nucleotide. The primer transfer is achieved through two novel intermediates, one a tension‐sensitive and functional polymerization conformation and a second non‐active state that may work as a fidelity check point for the proofreading reaction.     

Exonucleolytic proofreading by a mammalian DNA polymerase

Porcine liver DNA polymerase gamma contains exonuclease activity capable of digesting DNA in the 3'----5' direction, releasing deoxyribonucleoside 5'-monophosphates. The exonuclease activity excises 3'-terminal bases from both matched and mismatched primer termini, with a preference for mismatched bases. Under polymerization conditions, mismatch excision by the exonuclease occurs prior to polymerization by polymerase gamma, and this excision can be inhibited by adding to the reaction a high concentration of dNTP substrates and/or nucleoside 5'-monophosphates. In an M13mp2-based reversion assay for detecting single-base substitution errors, porcine liver polymerase gamma is highly accurate; the estimated base substitution error rate is less than one error for each 500,000 bases polymerized. Lower fidelity is observed using reaction conditions that inhibit the exonuclease activity, strongly suggesting that the exonuclease proofreads errors during polymerization. However, in a forward mutation assay capable of detecting all 12 mispairs at a variety of template positions, certain base substitution errors are readily detected even using unperturbed polymerization conditions. Thus, for some errors, polymerase gamma is not highly accurate, suggesting that proofreading is not equally active against all mispairs. To examine if the polymerase and exonuclease activities are physically as well as functionally associated, both activities were monitored during purification by four procedures, each based on a different separation principle. The two activities copurify during chromatography using phosphocellulose, heparin-agarose, or double-strand DNA-cellulose, and during velocity sedimentation in a glycerol gradient containing 0.5 M KCl. These results suggest that the polymerase and exonuclease activities are physically associated. It remains to be determined if they reside in the same subunit.     

Enzymatic determinants of DNA polymerase accuracy. Theory of coliphage T4 polymerase mechanisms

The specificity of base selection in DNA synthesis is affected by the DNA polymerase. This paper presents a detailed model of the enzymatic reaction steps involved in template-directed DNA synthesis by DNA polymerase possessing 3′ to 5′ exonuclease activity (the T4 coliphage enzyme and polymerases I, II and III of Escherichia coli are known examples). The central assumptions of the model imply that the strength of the hydrogen bonds formed between candidate bases and the template base provide the principal discrimination criteria for both the insertion and the “editing” excision of bases. We have performed a detailed analysis of this model, and of the stochastic, sequential, single-step process of insertion and excision, which it implies, and have compared the results with experimental data reported by Bessman et al. (1974a,b) for the DNA polymerase of T4. In the present version of the model, the presence of the editing exonuclease accounts for the enzyme's contribution to the accuracy of polymerization and all binding and reaction sites on the enzyme are insensitive to differences between bases or base-pairs. The Bessman data, describing the competitive incorporation of adenine and 2-aminopurine by five allelic polymerases and spanning a spectrum of mutation rates, are well fit by this theory, and permit us to predict the effects of changes in reaction conditions and in basepairing free energy on the outcome of similar experiments.     

Metal activation of synthetic and degradative activities of phi 29 DNA polymerase, a model enzyme for protein-primed DNA replication.

Analysis of metal activation on the synthetic and degradative activities of phi 29 DNA polymerase was carried out in comparison with T4 DNA polymerase and Escherichia coli DNA polymerase I (Klenow fragment). In the three DNA polymerases studied, both the polymerization and the 3'----5' exonuclease activity had clear differences in their metal ion requirements. The results obtained support the existence of independent metal binding sites for the synthetic and degradative activities of phi 29 DNA polymerase, according with the distant location of catalytic domains (N-terminal for the 3'----5' exonuclease and C-terminal for DNA polymerization) proposed for both Klenow fragment and phi 29 DNA polymerase. Furthermore, DNA competition experiments using phi 29 DNA polymerase suggested that the main differences observed in the metal usage to activate polymerization may be the consequence of metal-induced changes in the enzyme-DNA interactions, whose strength distinguishes processive and nonprocessive DNA polymerases. Interestingly, the initiation of DNA polymerization using a protein as a primer, a special synthetic activity carried out by phi 29 DNA polymerase, exhibited a strong preference for Mn2+ as metal activator. The molecular basis for this preference is mainly the result of a large increase in the affinity for dATP.     

Kinetic partitioning between the exonuclease and polymerase sites in DNA error correction

We present a kinetic partitioning mechanism by which the highly efficient 3'----5' exonuclease activity of T7 DNA polymerase maximizes its contribution to replication fidelity with minimal excision of correctly base-paired DNA. The elementary rate constants for the proposed mechanism have been measured directly from single-turnover experiments by using rapid chemical quench-flow techniques. The exonuclease activity of T7 DNA polymerase toward single-stranded DNA is quite fast (kx greater than 700 s-1). This rapid exonuclease is restrained with double-stranded DNA by a kinetic partitioning mechanism that favors the binding of the DNA to the polymerase site to prevent the rapid degradation of matched DNA and yet allows selective removal of mismatched DNAs. Both matched and mismatched DNAs bind tightly to the polymerase site, with approximately equal affinities, Kdp = 20 and 10 nM, respectively. Selective removal of the mismatch is governed by the rate of transfer of the DNA from the polymerase to the exonuclease site (kp----x). The rapid excision of matched DNA is limited by a slow transfer rate (kp----x = 0.2 s-1) from the polymerase to the exonuclease site relative to the rate of polymerization [kp = 300 s-1; Patel et al. (1991) Biochemistry (first of three papers in this issue)]. Removal of mismatched DNA is facilitated by its faster transfer rate (kp----x = 2.3 s-1) to the exonuclease site relative to the slow rate of polymerization over a mismatch [kpi = 0.012 s-1; Wong et al. (1991) Biochemistry (second of three papers in this issue)].(ABSTRACT TRUNCATED AT 250 WORDS)     

Processive proofreading and the spatial relationship between polymerase and exonuclease active sites of bacteriophage phi29 DNA polymerase.

phi29 DNA polymerase is a multifunctional enzyme, able to incorporate and to proofread misinserted nucleotides, maintaining a very high replication fidelity. Since both activities are functionally separated, a mechanism is needed to guarantee proper coordination between synthesis and degradation, implying movement of the DNA primer terminus between polymerization and 3'-5' exonuclease active sites. Using single-turnover conditions, we have demonstrated that phi29 DNA polymerase edits the polymerization errors using an intramolecular pathway; that is, the primer terminus travels from one active site to the other without dissociation from the DNA. On the other hand, by using chemical tags, we could infer a difference in length of only one nucleotide to contact the primer strand when it is in the polymerization mode versus the editing mode. Using the same approach, it was estimated that phi29 DNA polymerase covers a DNA region of ten nucleotides, as has been measured in other polymerases using different techniques.     

Substrate specificity and proposed structure of the proofreading complex of T7 DNA polymerase

Faithful replication of genomic DNA by high-fidelity DNA polymerases is crucial for the survival of most living organisms. While high-fidelity DNA polymerases favor canonical base pairs over mismatches by a factor of ∼1 × 10⁵, fidelity is further enhanced several orders of magnitude by a 3′–5′ proofreading exonuclease that selectively removes mispaired bases in the primer strand. Despite the importance of proofreading to maintaining genome stability, it remains much less studied than the fidelity mechanisms employed at the polymerase active site. Here we characterize the substrate specificity for the proofreading exonuclease of a high-fidelity DNA polymerase by investigating the proofreading kinetics on various DNA substrates. The contribution of the exonuclease to net fidelity is a function of the kinetic partitioning between extension and excision. We show that while proofreading of a terminal mismatch is efficient, proofreading a mismatch buried by one or two correct bases is even more efficient. Because the polymerase stalls after incorporation of a mismatch and after incorporation of one or two correct bases on top of a mismatch, the net contribution of the exonuclease is a function of multiple opportunities to correct mistakes. We also characterize the exonuclease stereospecificity using phosphorothioate-modified DNA, provide a homology model for the DNA primer strand in the exonuclease active site, and propose a dynamic structural model for the transfer of DNA from the polymerase to the exonuclease active site based on MD simulations.     

Kinetics and fidelity of polymerization by DNA polymerase III from Sulfolobus solfataricus

We have biochemically and kinetically characterized the polymerase and exonuclease activities of the third B-family polymerase (Dpo3) from the hyperthermophilic Crenarchaeon, Sulfolobus solfataricus (Sso). We have established through mutagenesis that despite incomplete sequence conservation, the polymerase and exonuclease active sites are functionally conserved in Dpo3. Using pre-steady-state kinetics, we can measure the fidelity of nucleotide incorporation by Dpo3 from the polymerase active site alone to be 10(3)-10(4) at 37 °C. The functional exonuclease proofreading active site will increase fidelity by at least 10(2), making Dpo3 comparable to other DNA polymerases in this family. Additionally, Dpo3's exonuclease activity is modulated by temperature, where a loss of promiscuous degradation activity can be attributed to a reorganization of the exonuclease domain when it is bound to primer-template DNA at high temperatures. Unexpectedly, the DNA binding affinity is weak compared with those of other DNA polymerases of this family. A comparison of the fidelity, polymerization kinetics, and associated functional exonuclease domain with those previously reported for other Sso polymerases (Dpo1 and Dpo4) illustrates that Dpo3 is a potential player in the proper maintenance of the archaeal genome.     

Spectroscopic analysis of polymerization and exonuclease proofreading by a high-fidelity DNA polymerase during translesion DNA synthesis

High fidelity DNA polymerases maintain genomic fidelity through a series of kinetic steps that include nucleotide binding, conformational changes, phosphoryl transfer, polymerase translocation, and nucleotide excision. Developing a comprehensive understanding of how these steps are coordinated during correct and pro-mutagenic DNA synthesis has been hindered due to lack of spectroscopic nucleotides that function as efficient polymerase substrates. This report describes the application of a non-natural nucleotide designated 5-naphthyl-indole-2′-deoxyribose triphosphate which behaves as a fluorogenic substrate to monitor nucleotide incorporation and excision during the replication of normal DNA versus two distinct DNA lesions (cyclobutane thymine dimer and an abasic site). Transient fluorescence and rapid-chemical quench experiments demonstrate that the rate constants for nucleotide incorporation vary as a function of DNA lesion. These differences indicate that the non-natural nucleotide can function as a spectroscopic probe to distinguish between normal versus translesion DNA synthesis. Studies using wild-type DNA polymerase reveal the presence of a fluorescence recovery phase that corresponds to the formation of a pre-excision complex that precedes hydrolytic excision of the non-natural nucleotide. Rate constants for the formation of this pre-excision complex are dependent upon the DNA lesion, and this suggests that the mechanism of exonuclease proofreading is regulated by the nature of the formed mispair. Finally, spectroscopic evidence confirms that exonuclease proofreading competes with polymerase translocation. Collectively, this work provides the first demonstration for a non-natural nucleotide that functions as a spectroscopic probe to study the coordinated efforts of polymerization and exonuclease proofreading during correct and translesion DNA synthesis.     

Evaluating the effects of enhanced processivity and metal ions on translesion DNA replication catalyzed by the bacteriophage T4 DNA polymerase

The fidelity of DNA replication is achieved in a multiplicative process encompassing nucleobase selection and insertion, removal of misinserted nucleotides by exonuclease activity, and enzyme dissociation from primer/templates that are misaligned due to mispairing. In this study, we have evaluated the effect of altering these kinetic processes on the dynamics of translesion DNA replication using the bacteriophage T4 replication apparatus as a model system. The effect of enhancing the processivity of the T4 DNA polymerase, gp43, on translesion DNA replication was evaluated using a defined in vitro assay system. While the T4 replicase (gp43 in complex with gp45) can perform efficient, processive replication using unmodified DNA, the T4 replicase cannot extend beyond an abasic site. This indicates that enhancing the processivity of gp43 does not increase unambiguously its ability to perform translesion DNA replication. Surprisingly, the replicase composed of an exonuclease-deficient mutant of gp43 was unable to extend beyond the abasic DNA lesion, thus indicating that molecular processes involved in DNA polymerization activity play the predominant role in preventing extension beyond the non-coding DNA lesion. Although neither T4 replicase complex could extend beyond the lesion, there were measurable differences in the stability of each complex at the DNA lesion. Specifically, the exonuclease-deficient replicase dissociates at a rate constant, k(off), of 1.1s(-1) while the wild-type replicase remains more stably associated at the site of DNA damage by virtue of a slower measured rate constant (k(off) 0.009s(-1)). The increased lifetime of the wild-type replicase suggests that idle turnover, the partitioning of the replicase from its polymerase to its exonuclease active site, may play an important role in maintaining fidelity. Further attempts to perturb the fidelity of the T4 replicase by substituting Mn(2+) for Mg(2+) did not significantly enhance DNA synthesis beyond the abasic DNA lesion. The results of these studies are interpreted with respect to current structural information of gp43 alone and complexed with gp45.     

Thermodynamic dissection of the polymerizing and editing modes of a DNA polymerase

DNA polymerases with intrinsic proofreading activity interact with DNA primer/templates in two distinct modes, corresponding to the complexes formed during the 5'-3' polymerization or 3'-5' editing of a nascent DNA chain. Thermodynamic measurements designed to quantify the energetic contributions of individual DNA-protein contacts in either the polymerizing or editing complexes are complicated by the fact that both species exist in solution and are not resolved in conventional DNA-protein binding assays. To overcome this problem, we have developed a new binding analysis that combines information from steady-state and time-resolved fluorescence experiments and uses the Klenow fragment of Escherichia coli DNA polymerase I (KF) and fluorescently labeled primer/template oligonucleotides as a model polymerase-DNA system. Steady-state fluorescence titrations are used to evaluate the overall affinity of KF for the primer/template, while time-resolved fluorescence anisotropy is used to quantify the equilibrium fractions of the primer/template bound in the polymerizing and editing modes. From a combined analysis of both data, the equilibrium constant and hence standard free energy change associated with each binding mode can be obtained unequivocally. This method is initially used to determine the equilibrium constants describing binding of a correctly base-paired primer/template to the 5'-3' polymerase and 3'-5' exonuclease sites of KF. It is then extended to quantify the extent to which these parameters are affected by the introduction of mismatches into the primer/template, and by rearrangement of specific side-chains in the exonuclease domain of the protein. While these perturbants were originally designed to demonstrate the utility of our new approach, they are also relevant in their own right since they have helped identify some hitherto unknown determinants of polymerase fidelity.     

DNA polymerase mutagenic bypass and proofreading of endogenous DNA lesions

DNA polymerases differentiate between correct and incorrect substrates during synthesis on undamaged DNA templates through the biochemical steps of base incorporation, primer-template extension and proofreading excision. Recent research examining DNA polymerase processing of abasic, alkylation and oxidative lesions is reviewed in light of these discrimination mechanisms. Inhibition of DNA synthesis results from correct polymerase discrimination against utilization of geometrically incorrect template bases or 3' terminal basepairs. The efficiency of translesion synthesis is thus related to the physical structure of the lesion containing DNA. However, variations in enzyme structure and kinetics result in translesion synthesis efficiencies that are also dependent upon the DNA polymerase. With a low probability, polymerase misinsertion events create a 3' lesion terminus which is geometrically favored over the correct lesion basepair, resulting in mutagenic translesion synthesis. For example, both polymerase alpha and polymerase beta appear to require the formation of a stable 3' primer-template structure for efficient abasic site translesion synthesis. However, the enzymes differ as to the precise molecular make-up of the stable DNA structure, resulting in different mutational specificities. Similar mechanisms may be applicable to oxidative damage, where mutational specificities dependent upon the DNA polymerase also have been observed. In vitro reaction conditions also influence DNA polymerase processing of lesions. Using an in vitro herpes simplex virus thymidine kinase (HSV-tk) gene forward mutation assay, we demonstrate that high dNTP substrate concentrations affect the mutagenic specificity of translesion synthesis using alkylated templates. The exonuclease-deficient Klenow polymerase error frequency for G-->A transition mutations using templates modified by N-ethyl-N-nitrosourea (ENU) was four-fold higher at 1000 microM [dNTP], relative to 50 microM [dNTP], consistent with an increased efficiency of extension of the etO6G.T mispair. Moreover, the frequency of other ENU-induced polymerase errors was suppressed when polymerase reactions contained 50 microM dNTP, relative to 1000 microM dNTP. The efficiency of proofreading as a polymerase error discrimination mechanism reflects a balance between the competing processes of 3'-->5' exonuclease removal of mispairs and polymerization of the next correct nucleotide. Polymerases that are devoid of a proofreading exonuclease generally display enhanced abasic site translesion synthesis relative to proofreading-proficient enzymes. In addition, the proofreading exonucleases of Escherichia coli Pol I and T4 DNA polymerases have been found to remove mispairs caused by abasic sites and oxidative lesions, respectively, resulting in lowered polymerase error rates. However, the magnitude of the exonuclease effect is small (less than 10-fold), and highly dependent upon the DNA polymerase-exonuclease. We have studied proofreading exonuclease removal of alkylation damage in the HSV-tk forward assay. We observed no significant reduction in the magnitude of the mutant frequency vs. dose-response curves when N-methyl-N-nitrosourea or ENU-treated templates were used in exonuclease-proficient Klenow polymerase reactions, as compared to the exonuclease-deficient polymerase reactions. Thus, available data suggest that proofreading excision of endogenous lesion mispairs does occur, but the efficiency is dependent upon the lesion and the DNA polymerase-exonuclease studied.     

Interaction of Escherichia coli DNA polymerase I with azidoDNA and fluorescent DNA probes: identification of protein-DNA contacts

The synthesis of an azidoDNA duplex and its use to photolabel DNA polymerases have been previously described (Gibson & Benkovic, 1987). We now present detailed experiments utilizing this azidoDNA photoprobe as a substrate for Escherichia coli DNA polymerase I (Klenow fragment) and the photoaffinity labeling of the protein. The azidoDNA duplex is an efficient substrate for both the polymerase and 3'----5' exonuclease activities of the enzyme. However, the hydrolytic degradation of the azido-bearing base is dramatically impaired. On the basis of the ability of these duplexes to photolabel the enzyme, we have determined that the protein contacts between five and seven bases of duplex DNA. Incubation of azidoDNA with the Klenow fragment in the presence of magnesium results in the in situ formation of a template-primer with the azido-bearing base bound at the polymerase catalytic site of the enzyme. Photolysis of this complex followed by proteolytic digestion and isolation of DNA-labeled peptides results in the identification of a single residue modified by the photoreactive DNA substrate. We identify Tyr766 as the modified amino acid and thus localize the catalytic site for polymerization in the protein. A mansyl-labeled DNA duplex has been prepared as a fluorescent probe of protein structure. This has been utilized to determine the location of the primer terminus when bound to the Klenow fragment. When the duplex contains five unpaired bases in the primer strand of the duplex, the primer terminus resides predominantly at the exonuclease catalytic site of the enzyme. Removal of the mismatched bases by the exonuclease activity of the enzyme yields a binary complex with the primer terminus now bound predominantly at the polymerase active site. Data are presented which suggest that the rate-limiting step in the exonuclease activity of the enzyme is translocation of the primer terminus from polymerase to exonuclease catalytic sites.     

Processive replication of single-stranded DNA templates by the herpes simplex virus-induced DNA polymerase

The DNA polymerase encoded by herpes simplex virus 1 consists of a single polypeptide of Mr 136,000 that has both DNA polymerase and 3'----5' exonuclease activities; it lacks a 5'----3' exonuclease. The herpes polymerase is exceptionally slow in extending a synthetic DNA primer annealed to circular single-stranded DNA (turnover number approximately 0.25 nucleotide). Nevertheless, it is highly processive because of its extremely tight binding to a primer terminus (Kd less than 1 nM). The single-stranded DNA-binding protein from Escherichia coli greatly stimulates the rate (turnover number approximately 4.5 nucleotides) by facilitating the efficient binding to and extension of the DNA primers. Synchronous replication by the polymerase of primed single-stranded DNA circles coated with the single-stranded DNA-binding protein proceeds to the last nucleotide of available 5.4-kilobase template without dissociation, despite the 20-30 min required to replicate the circle. Upon completion of synthesis, the polymerase is slow in cycling to other primed single-stranded DNA circles. ATP (or dATP) is not required to initiate or sustain highly processive synthesis. The 3'----5' exonuclease associated with the herpes DNA polymerase binds a 3' terminus tightly (Km less than 50 nM) and is as sensitive as the polymerase activity to inhibition by phosphonoacetic acid (Ki approximately 4 microM), suggesting close communication between the polymerase and exonuclease sites.     

NaF and mononucleotides as inhibitors of 3'-5'-exonuclease activity and stimulators of polymerase activity of E. coli DNA polymerase I Klenow fragment

It has been shown that, in the absence of dATP in the poly(dT).oligo(dA) template-primer complex, the rate of primer cleavage by the E. coli DNA polymerase I Klenow fragment equals 4% of polymerization rate, while in the presence of dATP it equals as much as 50-60%. NaF and NMP taken separately inhibit exonuclease cleavage of oligo(dA) both with and without dATP. The addition of NaF (5-10 mM) or NMP (5-20 mM) increases the absolute increment of polymerization rate 5-9-fold relative to the absolute decrement of the rate of nuclease hydrolysis of primer. This proves the assumption that not more than 10-20% of primer molecules, interacting with the exonuclease center of polymerase, are cleaved by the enzyme. Presumably, NaF and nucleotides disturb the coupling of the 3'-end of oligonucleotide primer to the exonuclease center of the enzyme. As the primers mostly form complexes with the polymerizing center, the reaction of polymerization is activated.     

The J-helix of Escherichia coli DNA polymerase I (Klenow fragment) regulates polymerase and 3'- 5'-exonuclease functions

To assess the functional importance of the J-helix region of Escherichia coli DNA polymerase I, we performed site-directed mutagenesis of the following five residues: Asn-675, Gln-677, Asn-678, Ile-679, and Pro-680. Of these, the Q677A mutant is polymerase-defective with no change in its exonuclease activity. In contrast, the N678A mutant has unchanged polymerase activity but shows increased mismatch-directed exonuclease activity. Interestingly, mutation of Pro-680 has a Q677A-like effect on polymerase activity and an N678A-like effect on the exonuclease activity. Mutation of Pro-680 to Gly or Gln results in a 10-30-fold reduction in k(cat) on homo- and heteropolymeric template-primers, with no significant change in relative DNA binding affinity or K(m)((dNTP)). The mutants P680G and P680Q also showed a nearly complete loss in the processive mode of DNA synthesis. Since the side chain of proline is generally non-reactive, mutation of Pro-680 may be expected to alter the physical form of the J-helix itself. The biochemical properties of P680G/P680Q together with the structural observation that J-helix assumes helical or coiled secondary structure in the polymerase or exonuclease mode-bound DNA complexes suggest that the structural alteration in the J-helix region may be responsible for the controlled shuttling of DNA between the polymerase and the exonuclease sites.     

The bacteriophage phi 29 DNA polymerase, a proofreading enzyme.

The bacteriophage phi 29 DNA polymerase, involved both in the protein-primed initiation and elongation steps of the viral DNA replication, displays a very processive 3',5'-exonuclease activity acting preferentially on single-stranded DNA. This exonucleolytic activity showed a marked preference for excision of a mismatched versus a correctly paired 3' terminus. These characteristics enable the phi 29 DNA polymerase to act as a proofreading enzyme. A comparative analysis of the wild-type phi 29 DNA polymerase and a mutant lacking 3',5'-exonuclease activity indicated that a productive coupling between the exonuclease and polymerase activities is necessary to prevent fixation of polymerization errors. Based on these data, the phi 29 DNA polymerase, a model enzyme for protein-primed DNA replication, appears to share the same mechanism for the editing function as that first proposed for T4 DNA polymerase and Escherichia coli DNA polymerase I on the basis of functional and structural studies.     

Selective oxidation of the exonuclease domain of bacteriophage T7 DNA polymerase

Bacteriophage T7 DNA polymerase, the product of gene 5 of the phage, has both polymerase and single-and double-stranded DNA 3'-to 5'-exonuclease activities. The exonuclease activities can be inactivated selectively by an oxidation reaction that requires molecular oxygen, a reducing agent, and iron at a concentration less than or equimolar to that of the gene 5 protein. Both exonuclease activities can be diminished by several thousandfold, with only a small decline in the polymerase activity. Escherichia coli thioredoxin, an accessory protein that binds tightly to the gene 5 protein and increases the processivity of the polymerization reaction, has no effect on the rate of oxidation. We propose that iron binds specifically to the exonuclease domain and, in the presence of molecular oxygen and a reducing agent, generates reactive oxygen species that selectively modify amino acid residues essential for the exonuclease activities.     

The fidelity of DNA synthesis catalyzed by derivatives of Escherichia coli DNA polymerase I

The fidelity of DNA synthesis by an exonuclease-proficient DNA polymerase results from the selectivity of the polymerization reaction and from exonucleolytic proofreading. We have examined the contribution of these two steps to the fidelity of DNA synthesis catalyzed by the large Klenow fragment of Escherichia coli DNA polymerase I, using enzymes engineered by site-directed mutagenesis to inactivate the proofreading exonuclease. Measurements with two mutant Klenow polymerases lacking exonuclease activity but retaining normal polymerase activity and protein structure demonstrate that the base substitution fidelity of polymerization averages one error for each 10,000 to 40,000 bases polymerized, and can vary more than 30-fold depending on the mispair and its position. Steady-state enzyme kinetic measurements of selectivity at the initial insertion step by the exonuclease-deficient polymerase demonstrate differences in both the Km and the Vmax for incorrect versus correct nucleotides. Exonucleolytic proofreading by the wild-type enzyme improves the average base substitution fidelity by 4- to 7-fold, reflecting efficient proofreading of some mispairs and less efficient proofreading of others. The wild-type polymerase is highly accurate for -1 base frameshift errors, with an error rate of less than or equal to 10(-6). The exonuclease-deficient polymerase is less accurate, suggesting that proofreading also enhances frameshift fidelity. Even without a proofreading exonuclease, Klenow polymerase has high frameshift fidelity relative to several other DNA polymerases, including eucaryotic DNA polymerase-alpha, an exonuclease-deficient, 4-subunit complex whose catalytic subunit is almost three times larger. The Klenow polymerase has a large (46 kDa) domain containing the polymerase active site and a smaller (22 kDa) domain containing the active site for the 3'----5' exonuclease. Upon removal of the small domain, the large polymerase domain has altered base substitution error specificity when compared to the two-domain but exonuclease-deficient enzyme. It is also less accurate for -1 base errors at reiterated template nucleotides and for a 276-nucleotide deletion error. Thus, removal of a protein domain of a DNA polymerase can affect its fidelity.