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PubMed2005

Nuclear DNA helicase II (RNA helicase A) interacts with Werner syndrome helicase and stimulates its exonuclease activity.

Nuclear DNA helicase II (NDH II), alternatively named RNA helicase A, is involved in transcription and RNA processing. Here, we report that NDH II interacts with the Werner syndrome helicase WRN, an enzyme associated with premature aging and predisposition to tumorigenesis. NDH II was co-purified with WRN, DNA polymerase delta, and replication protein A (70 kDa) during several steps of conventional column chromatography. Co-immunoprecipitations revealed an association between NDH II, WRN, and polymerase delta. We demonstrate a direct protein-protein interaction between WRN and NDH II that is mediated by the N-terminal double-strand RNA-binding domain II and C-terminal RGG box of NDH II and the N-terminal exonuclease domain of WRN. WRN inhibited the DNA-dependent NTPase and DNA helicase activities of NDH II. On the other hand, the 3' --> 5' exonuclease activity of WRN was increased by the presence of NDH II. NDH II directly stimulated the exonuclease domain of WRN, whereas the exonuclease domain of WRN suppressed the DNA-dependent (but not RNA-dependent) ATPase activity of NDH II. These results suggest that the double-strand RNA-binding domain II and RGG box of NDH II together form a protein-protein interaction surface that contacts the exonuclease domain of WRN. Furthermore, NDH II enhanced the degradation of D-loop DNA by the WRN exonuclease. Taken together, these results suggest that NDH II plays a role in promoting the DNA processing function of WRN, which in turn might be necessary for maintaining genomic stability.

Animals↗
PubMed1999

Identification of RNase T as a high-copy suppressor of the UV sensitivity associated with single-strand DNA exonuclease deficiency in Escherichia coli.

There are three known single-strand DNA-specific exonucleases in Escherichia coli: RecJ, exonuclease I (ExoI), and exonuclease VII (ExoVII). E. coli that are deficient in all three exonucleases are abnormally sensitive to UV irradiation, most likely because of their inability to repair lesions that block replication. We have performed an iterative screen to uncover genes capable of ameliorating the UV repair defect of xonA (ExoI-) xseA (ExoVII-) recJ triple mutants. In this screen, exonuclease-deficient cells were transformed with a high-copy E. coli genomic library and then irradiated; plasmids harvested from surviving cells were used to seed subsequent rounds of transformation and selection. After several rounds of selection, multiple plasmids containing the rnt gene, which encodes RNase T, were found. An rnt plasmid increased the UV resistance of a xonA xseA recJ mutant and uvrA and uvrC mutants; however, it did not alter the survival of xseA recJ or recA mutants. RNase T also has amino acid sequence similarity to other 3' DNA exonucleases, including ExoI. These results suggest that RNase T may possess a 3' DNase activity capable of substituting for ExoI in the recombinational repair of UV-induced lesions.

Bacterial Proteins↗
PubMed1997

Prokaryotic 5'-3' exonucleases share a common core structure with gamma-delta resolvase.

The three dimensional crystal structure of T5 5'-3' exonuclease was compared with that of two other members of the 5'-3' exonuclease family: T4 ribonuclease H and the N-terminal domain of Thermus aquaticus DNA polymerase I. Though these structures were largely similar, some regions of these enzymes show evidence of significant molecular flexibility. Previous sequence analysis had suggested the existence of a helix-hairpin-helix motif in T5 exonuclease, but a distinct, though related structure is actually found to occur. The entire T5 exonuclease structure was then compared with all the structures in the complete Protein Data Bank and an unexpected similarity with gamma-delta (gamma delta) resolvase was observed. 5'-3' exonucleases and gamma delta resolvase are enzymes involved in carrying out quite different manipulations on nucleic acids. They appear to be unrelated at the primary sequence level, yet the fold of the entire catalytic domain of gamma delta resolvase is contained within that of the 5'-3'exonuclease. Different large-scale helical structures are used by both families to form DNA binding sites.

Amino Acid Sequence↗
PubMed2000

Escherichia coli exonuclease III enhances long PCR amplification of damaged DNA templates.

Recent development of the long PCR technology has provided an invaluable tool in many areas of molecular biology. However, long PCR amplification fails whenever the DNA template is imperfectly preserved. We report that Escherichia coli exonuclease III, a major repair enzyme in bacteria, strikingly improves the long PCR amplification of damaged DNA templates. Escherichia coli exonuclease III permitted or improved long PCR amplification with DNA samples submitted to different in vitro treatments known to induce DNA strand breaks and/or apurinic/apyrimidinic (AP) sites, including high temperature (99 degrees C), depurination at low pH and near-UV radiation. Exonuclease III also permitted or improved amplification with DNA samples that had been isolated several years ago by the phenol/chloroform method. Amelioration of long PCR amplification was achieved for PCR products ranging in size from 5 to 15.4 kb and with DNA target sequences located either within mitochondrial DNA or the nuclear genome. Exonuclease III increased the amplification of damaged templates using either rTth DNA polymerase alone or rTth plus Vent DNA polymerases or TAQ: plus PWO: DNA polymerases. However, exonuclease III could not improve PCR amplification from extensively damaged DNA samples. In conclusion, supplementation of long PCR mixes with E.COLI: exonuclease III may represent a major technical advance whenever DNA samples have been partly damaged during isolation or subsequent storage.

Animals↗
PubMed2001

Real-time observation of a single DNA digestion by lambda exonuclease under a fluorescence microscope field.

A fluorescence microscopy technique has been developed to visualize the behavior of individual DNA and protein molecules. Real-time direct observation of a single DNA molecule can be used to investigate the dynamics of DNA-protein interactions, such as the DNA digestion reaction by lambda exonuclease. In conventional methods it is impossible to analyze the dynamics of an individual lambda exonuclease molecule on a DNA because they can only observe the average behavior of a number of exonuclease molecules. Observation of a single molecule, on the other hand, can reveal processivity and binding rate of an individual exonuclease molecule. To evaluate the dynamics of lambda exonuclease, a stained lambda DNA molecule with one biotinylated terminal was fixed on an avidin-coated coverslip and straightened using a d.c. electric field. Microscopic observation of digestion of a straightened DNA molecule by lambda exonuclease revealed that the DNA digestion rate was approximately 1000 bases/s and also demonstrated high processivity.

Avidin↗
PubMed1988

Analysis of the recE locus of Escherichia coli K-12 by use of polyclonal antibodies to exonuclease VIII.

Exonuclease VIII (exoVIII) of Escherichia coli has been purified from a strain carrying a plasmid-encoded recE gene by using a new procedure. This procedure yielded 30 times more protein per gram of cells, and the protein had a twofold higher specific activity than the enzyme purified by the previously published procedure (J. W. Joseph and R. Kolodner, J. Biol. Chem. 258:10411-10417, 1983). The sequence of the 12 N-terminal amino acids was also obtained and found to correspond to one of the open reading frames predicted from the nucleic acid sequence of the recE region of Rac (C. Chu, A. Templin, and A. J. Clark, manuscript in preparation). Polyclonal antibodies directed against purified exoVIII were also prepared. Cell-free extracts prepared from strains containing a wide range of chromosomal- or plasmid-encoded point, insertion, and deletion mutations which result in expression of exoVIII were examined by Western blot (immunoblot) analysis. This analysis showed that two point sbcA mutations (sbcA5 and sbcA23) and the sbc insertion mutations led to the synthesis of the 140-kilodalton (kDa) polypeptide of wild-type exoVIII. Plasmid-encoded partial deletion mutations of recE reduced the size of the cross-reacting protein(s) in direct proportion to the size of the deletion, even though exonuclease activity was still present. The analysis suggests that 39 kDa of the 140-kDa exoVIII subunit is all that is essential for exonuclease activity. One of the truncated but functional exonucleases (the pRAC3 exonuclease) has been purified and confirmed to be a 41-kDa polypeptide. The first 18 amino acids from the N terminus of the 41-kDa pRAC3 exonuclease were sequenced and fond to correspond to one of the translational start signals predicted from the nucleotide sequence of radC (Chu et al., in preparation).

Alleles↗
PubMed1991

Polymerization activity of an alpha-like DNA polymerase requires a conserved 3'-5' exonuclease active site.

Most DNA polymerases are multifunctional proteins that possess both polymerizing and exonucleolytic activities. For Escherichia coli DNA polymerase I and its relatives, polymerase and exonuclease activities reside on distinct, separable domains of the same polypeptide. The catalytic subunits of the alpha-like DNA polymerase family share regions of sequence homology with the 3'-5' exonuclease active site of DNA polymerase I; in certain alpha-like DNA polymerases, these regions of homology have been shown to be important for exonuclease activity. This finding has led to the hypothesis that alpha-like DNA polymerases also contain a distinct 3'-5' exonuclease domain. We have introduced conservative substitutions into a 3'-5' exonuclease active site homology in the gene encoding herpes simplex virus DNA polymerase, an alpha-like polymerase. Two mutants were severely impaired for viral DNA replication and polymerase activity. The mutants were not detectably affected in the ability of the polymerase to interact with its accessory protein, UL42, or to colocalize in infected cell nuclei with the major viral DNA-binding protein, ICP8, suggesting that the mutation did not exert global effects on protein folding. The results raise the possibility that there is a fundamental difference between alpha-like DNA polymerases and E. coli DNA polymerase I, with less distinction between 3'-5' exonuclease and polymerase functions in alpha-like DNA polymerases.

Amino Acid Sequence↗
PubMed1999

Different regulation of the p53 core domain activities 3'-to-5' exonuclease and sequence-specific DNA binding.

In this study we further characterized the 3'-5' exonuclease activity intrinsic to wild-type p53. We showed that this activity, like sequence-specific DNA binding, is mediated by the p53 core domain. Truncation of the C-terminal 30 amino acids of the p53 molecule enhanced the p53 exonuclease activity by at least 10-fold, indicating that this activity, like sequence-specific DNA binding, is negatively regulated by the C-terminal basic regulatory domain of p53. However, treatments which activated sequence-specific DNA binding of p53, like binding of the monoclonal antibody PAb421, which recognizes a C-terminal epitope on p53, or a higher phosphorylation status, strongly inhibited the p53 exonuclease activity. This suggests that at least on full-length p53, sequence-specific DNA binding and exonuclease activities are subject to different and seemingly opposing regulatory mechanisms. Following up the recent discovery in our laboratory that p53 recognizes and binds with high affinity to three-stranded DNA substrates mimicking early recombination intermediates (C. Dudenhoeffer, G. Rohaly, K. Will, W. Deppert, and L. Wiesmueller, Mol. Cell. Biol. 18:5332-5342), we asked whether such substrates might be degraded by the p53 exonuclease. Addition of Mg2+ ions to the binding assay indeed started the p53 exonuclease and promoted rapid degradation of the bound, but not of the unbound, substrate, indicating that specifically recognized targets can be subjected to exonucleolytic degradation by p53 under defined conditions.

Animals↗
PubMed1992

3'-->5' exonuclease in Drosophila mitochondrial DNA polymerase. Substrate specificity and functional coordination of nucleotide polymerization and mispair hydrolysis.

A mispair-specific 3'-->5' exonuclease copurifies quantitatively with the near-homogeneous Drosophila gamma polymerase (Kaguni, L.S., and Olson, M.W. (1989) Proc. Natl. Acad. Sci. U.S.A. 86, 6469-6473). The exonuclease and polymerase exhibit similar reaction requirements and optima, suggesting functional coordination of their activities. Under nonpolymerization conditions, the 3'-->5' exonuclease hydrolyzes 3'-terminal mispairs approximately 15-fold more efficiently than 3'-terminal base pairs on primed single-stranded DNA substrates, whereas it does not discriminate between any of three specific mispairs (dAMP:dAMP;dGMP:dGMP; dGMP:dAMP). Under polymerization conditions, gamma polymerase does not extend a 3'-terminal mispair from the "stationary" state, even in the presence of a large excess of the next correct nucleotide. Instead, 3'-terminal mispairs are hydrolyzed quantitatively by the 3'-->5' exonuclease over the reaction time course. During DNA synthesis by gamma polymerase in the "polymerization" mode, limited misincorporation and subsequent mispair extension do occur. Here, it appears that misincorporation and not mispair extension is rate-limiting. Template-primer challenge experiments suggest that the mechanism of template-primer transfer from the 3'-->5' exonuclease active site to the DNA polymerase active site is intermolecular; transfer from the exonuclease to polymerase mode appears to require dissociation and reassociation of mitochondrial DNA polymerase.

Adenosine Monophosphate↗
PubMed1992

A DNA exonuclease induced during meiosis of Schizosaccharomyces pombe.

In meiotic cells of the fission yeast Schizosaccharomyces pombe, a DNA exonuclease activity increased approximately 5-fold after premeiotic S-phase and decreased to the initial level before the meiotic divisions. We have purified this activity, designated exonuclease I, to near homogeneity. The activity co-purified with a polypeptide with an apparent molecular weight of 36,000. With a linear double-stranded DNA substrate, exonuclease I degraded only the 5'-ended strand from each end to produce 3'-single-stranded tails. The enzyme also acted on nicked circular DNA with comparable affinity. The meiotic induction of exonuclease I and its mode of action, similar to that of recombination-promoting exonucleases from bacteria, suggest that exonuclease I is involved in meiotic homologous recombination in S. pombe.

Chromatography, Affinity↗
PubMed1990

Metal binding to DNA polymerase I, its large fragment, and two 3',5'-exonuclease mutants of the large fragment.

DNA polymerase I (Pol I) is an enzyme of DNA replication and repair containing three active sites, each requiring divalent metal ions such as Mg2+ or Mn2+ for activity. As determined by EPR and by 1/T1 measurements of water protons, whole Pol I binds Mn2+ at one tight site (KD = 2.5 microM) and approximately 20 weak sites (KD = 600 microM). All bound metal ions retain one or more water ligands as reflected in enhanced paramagnetic effects of Mn2+ on 1/T1 of water protons. The cloned large fragment of Pol I, which lacks the 5',3'-exonuclease domain, retains the tight metal binding site with little or no change in its affinity for Mn2+, but has lost approximately 12 weak sites (n = 8, KD = 1000 microM). The presence of stoichiometric TMP creates a second tight Mn2+ binding site or tightens a weak site 100-fold. dGTP together with TMP creates a third tight Mn2+ binding site or tightens a weak site 166-fold. The D424A (the Asp424 to Ala) 3',5'-exonuclease deficient mutant of the large fragment retains a weakened tight site (KD = 68 microM) and has lost one weak site (n = 7, KD = 3500 microM) in comparison with the wild-type large fragment, and no effect of TMP on metal binding is detected. The D355A, E357A (the Asp355 to Ala, Glu357 to Ala double mutant of the large fragment of Pol I) 3',5'-exonuclease-deficient double mutant has lost the tight metal binding site and four weak metal binding sites. The binding of dGTP to the polymerase active site of the D355A,E357A double mutant creates one tight Mn2+ binding site with a dissociation constant (KD = 3.6 microM), comparable with that found on the wild-type enzyme, which retains one fast exchanging water ligand. Mg2+ competes at this site with a KD of 100 microM. It is concluded that the single tightly bound Mn2+ on Pol I and a weakly bound Mn2+ which is tightened 100-fold by TMP are at the 3',5'-exonuclease active site and are essential for 3',5'-exonuclease activity, but not for polymerase activity. Additional weak Mn2+ binding sites are detected on the 3',5'-exonuclease domain, which may be activating, and on the polymerase domain, which may be inhibitory. The essential divalent metal activator of the polymerase reaction requires the presence of the dNTP substrate for tight metal binding indicating that the bound substrate coordinates the metal.(ABSTRACT TRUNCATED AT 400 WORDS)

Amino Acid Sequence↗
PubMed1989

DNA polymerase gamma from Xenopus laevis. II. A 3'----5' exonuclease is tightly associated with the DNA polymerase activity.

Xenopus laevis DNA polymerase gamma co-purifies with a tightly associated 3'----5' exonuclease. The purified enzyme lacks 5'----3' exonuclease and endonuclease activity. The ratio of the 3'----5' exonuclease activity to DNA polymerase gamma activity remains constant over the final three chromatographic procedures. In addition, these activities co-sediment under partially denaturing conditions in the presence of ethylene glycol. The associated 3'----5' exonuclease activity removes a terminally mismatched nucleotide more rapidly than a correctly base-paired 3'-terminal residue, as expected if this exonuclease has a proofreading function. The 3'----5' exonuclease has the ability to release a terminal phosphorothioated nucleotide, a property shared with T4 DNA polymerase, but not with Escherichia coli DNA polymerase I.

Animals↗
PubMed1983

The 3'-5' proofreading exonuclease of bacteriophage T4 DNA polymerase is stimulated by other T4 DNA replication proteins.

The bacteriophage T4 DNA polymerase has an intrinsic 3'-5' proofreading exonuclease activity that plays a central role in determining the fidelity of T4 DNA replication. In order to monitor this activity, we have measured the rate at which the polymerase decreases the size of a double-stranded DNA substrate in the absence of deoxyribonucleoside triphosphates. With this assay, we find that the addition of the polymerase accessory proteins, 45 protein and 44/62 protein, increases the rate at which the polymerase-associated exonuclease digests the DNA substrate 3- to 4-fold. This stimulation requires the continuous hydrolysis of ATP catalyzed by the accessory protein complex. When added alone, the T4 helix-destabilizing protein, 32 protein, inhibits the exonuclease rate at high concentrations (greater than 100 micrograms/ml), while stimulating about 3-fold at low concentrations. The 32 protein and the accessory proteins together increase the exonuclease rate 8- to 10-fold above that found for the polymerase alone. The bacteriophage T7 DNA polymerase displays a similar 3'-5' exonuclease activity, but this exonuclease is not stimulated by any of the T4 replication proteins. It therefore appears that specific protein-protein interactions are involved.

Adenosine Triphosphate↗
PubMed1993

A reverse DNA strand exchange mediated by recA protein and exonuclease I. The generation of apparent DNA strand breaks by recA protein is explained.

The combined action of exonuclease I and recA protein leads to a kind of reverse DNA strand exchange in which joint molecules formed on the "wrong" or distal end of a linear duplex in the presence of ATP are stabilized by exonuclease I degradation of the displaced (+) strand. Continued pairing and degradation of the displaced strand leads to strand exchange that appears to progress with a polarity opposite that of the normal recA protein promoted reaction (i.e. 3'-5' with respect to the (+) strand). However, in contrast to the normal 5'-3' strand exchange, the displaced strand is completely degraded in the process. When the linear duplex DNA substrate has a heterologous region at the 5' (proximal) end, the major product (described in a previous study (Bedale, W. A., Inman, R. B., and Cox, M. M. (1991) J. Biol. Chem. 266, 6499-6510)) is a circular duplex DNA molecule with a double-stranded tail whose length corresponds closely to the heterologous segment of the substrate. The origin of this product is here shown to be the result of the exonuclease activity of exonuclease I (either added exogenously or present as a trace contaminant of recA protein or SSB protein preparations), as opposed to endonucleolytic or mechanical breakage. The levels of exonuclease I required to generate these products are sufficiently low that they are undetected by assays for exonuclease contamination in recA protein preparations. These results demonstrate that the interplay of recA protein with other enzymes can have a profound effect on both the mechanism and outcome of recA protein-promoted DNA strand exchange. They also demonstrate that the (+) strand of the duplex DNA substrate is at least transiently displaced in recA protein-mediated pairing even when joint molecules are limited to the distal end.

Bacteriophage M13↗
PubMed1998

Oligomeric protein structure of beef spleen exonuclease.

A one-step purification of beef spleen exonuclease in the form of a DNA-enzyme complex is described. The purity of the exonuclease was verified by two-dimensional gel electrophoresis. It possesses molecular mass 160 kDa and pI 6.92. The one-dimensional sodium dodecyl sulfate gel after reduction with beta-mercaptoethanol suggests that the 160-kDa exonuclease consists of four polypeptide chains of two different types with molecular masses 55 and 25 kDa. The tetrameric structure of the exonuclease is supported by intermolecular disulfide bonds, and their partial reduction leads to the formation of one stable intermediate with molecular mass 80 kDa formed by binding one 55-kDa with one 25-kDa subunit into a dimer. During two-dimensional gel electrophoresis, the dimer showed pI 6.92 while monomers showed pI 6.78 for the 55-kDa and pI 6.29 for the 25-kDa. Two other intermediate states of two big and one small (135 kDa) and two small and one big subunit (105 kDa) were also visualized. They are unstable and easily dissociated into one 80-kDa dimer and either one 55-kDa or one 25-kDa monomer. The immunoblotting analysis with specific polyclonal antibodies against 160-kDa protein confirmed the subunit structure of the exonuclease. It was found that both monomers are glycosylated.

Animals↗
PubMed1997

An invariant lysine residue is involved in catalysis at the 3'-5' exonuclease active site of eukaryotic-type DNA polymerases.

A lysine residue, contained in the motif "Kx2h", has been invariantly found in the eukaryotic-type (family B) class of DNA-dependent DNA polymerases with a proofreading function. The importance of this lysine has been assessed by site-directed mutagenesis in the corresponding residue (Lys143) of phi29 DNA polymerase. Substitution of this residue either by arginine or isoleucine severely impaired the catalytic efficiency of the 3'-5' exonuclease activity, giving a characteristic distributive pattern that contrasts with the processive pattern displayed by the wild-type phi29 DNA polymerase. Exonuclease assays carried out in the presence of a DNA trap, together with direct analysis of enzyme/ssDNA interaction, allowed us to conclude that this altered pattern was due to a reduction in the catalytic rate of these mutants, but not to a weakened association with ssDNA. These phenotypes indicate that the lysine residue of motif Kx2h plays an auxiliary role in catalysis of the exonuclease reaction, in very good agreement with recent crystallographic data showing that the lysine homologue of T4 DNA polymerase is indirectly involved in metal binding at the 3'-5' exonuclease active site. In agreement with a critical role in proofreading, substitution of Lys143 of phi29 DNA polymerase by arginine or isoleucine produced mutator enzymes that displayed a high frequency of misincorporation. Mutants at Lys143 also showed a reduced DNA polymerization capacity, but only when DNA synthesis was coupled to strand-displacement, an intrinsic property of phi29 DNA polymerase that is specifically affected by mutations at residues directly or indirectly involved in metal binding at the 3'-5' exonuclease active site.

Amino Acid Sequence↗
PubMed1988

An endo-exonuclease activity of yeast that requires a functional RAD52 gene.

Extracts of Rad+ and radiation-sensitive (rad) mutants of the yeast Saccharomyces cerevisiae were examined for total Mg2+-dependent alkaline deoxyribonuclease activity and the presence of a nuclease that crossreacts immunologically with an antiserum raised against an endo-exonuclease from Neurospora crassa, an enzyme exhibiting both deoxyribo- and ribonuclease activities. No significant differences were observed in total deoxyribonuclease activity between Rad+ and rad mutants. The antibody precipitable activity, however, was found to be 30%-40% of the total alkaline deoxyribonuclease activity in logarithmically growing Rad+ cells. Extracts of stationary phase cells were lacking in antibody precipitable activity. Using immunoblot methods, a 72 kDa crossreacting protein was identified from logarithmically growing cells that was absent from stationary phase cells. In all radiation-sensitive mutants examined, except rad52, at least 20% of total activity was precipitable. Extracts from logarithmically growing rad52 mutants, including a rad52::LEU2 insertion mutant, exhibited less than 10% of the Rad+ precipitable activity; however, some crossreacting material was detected. Although, the level of endo-exonuclease activity is influenced by the RAD52 gene, it is not the product of this gene. The total deoxyribonuclease and the antibody precipitable endo-exonuclease activities were also followed during meiosis. Unlike the Rad+ strain which had previously been shown to have increased levels of total and immunoprecipitable endo-exonuclease as cells underwent meiosis, the rad52 mutant exhibited no increases in either category of nuclease activity. Given the importance of the RAD52 gene in repair, recombination and mutagenesis, the endo-exonuclease may be a significant component of these processes.

DNA Repair↗
PubMed1992

The 3'-->5' exonuclease associated with HeLa DNA polymerase epsilon.

The 3'-->5' exonuclease activity of highly purified large form of human DNA polymerase epsilon was studied. The activity removes mononucleotides from the 3' end of an oligonucleotide with a non-processive mechanism and leaves 5'-terminal trinucleotide non-hydrolyzed. This is the case both with single-stranded oligonucleotides and with oligonucleotides annealed to complementary regions of M13DNA. However, the reaction rates with single-stranded oligonucleotides are at least ten-fold when compared to those with completely base-paired oligonucleotides. Conceivably, mismatched 3' end of an oligonucleotide annealed to M13DNA is rapidly removed and the hydrolysis is slowed down when double-stranded region is reached. The preferential removal of a non-complementary 3' end and the nonprocessive mechanism are consistent with anticipated proofreading function. In addition to the 3'-->5' exonuclease activity, an 5'-->3' exonuclease activity is often present even in relatively highly purified DNA polymerase epsilon preparates suggesting that such an activity may be an essential component for the action of this enzyme in vivo. Contrary to the 3'-->5' exonuclease activity, the 5'-->3' exonuclease is separable from the polymerase activity.

Base Sequence↗