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D P Millar

Publications and source records attributed to D P Millar.

At least 19 recordsLinked to original sources
PubMed1998

Structural dynamics of HIV-1 Rev and its complexes with RRE and 5S RNA.

The Rev protein of the human immunodeficiency virus type 1 (HIV-1) has been studied by time-resolved fluorescence spectroscopy. The single tryptophan residue of Rev, Trp45, located within the arginine-rich RNA-binding domain of the protein, was utilized as an intrinsic spectroscopic probe. In addition, five peptides spanning different lengths of the arginine-rich domain, each containing the tryptophan residue, and two C-terminal deletion mutants of Rev, Rev M9 delta 14 and Rev M11 delta 14, were examined. Rev M9 delta 14 lacks residues 68-112 whereas Rev M11 delta 14 is missing residues 92-112 of the C-terminus of Rev. The fluorescence decay of Trp45 in wild-type Rev was resolved into four discrete lifetime components, and decay-associated spectra (DAS) were obtained for each component. The fluorescence decays of all five peptides and Rev M9 delta 14 were resolved into three lifetime components. The fluorescence decay of Rev M11 delta 14 was resolved into four components similar to those found for wild-type Rev. These results indicate that the activation domain (residues 78-93), present in wild-type Rev and Rev M11 delta 14, induced a unique tryptophan environment, characterized by a short-lived, blue-shifted emission, attributed to higher order assembly of Rev. In addition, fluorescence anisotropy decay data obtained for wild-type Rev and the two C-terminal deletion mutants also indicate that the activation domain mediates self-association of Rev. Based on the anisotropy decay results for wild-type Rev, the distribution of oligomers is independent of salt concentration. The average fluorescence lifetime of Trp45 was reduced upon complexation of Rev with a 40-mer fragment of the Rev response element containing the minimal element for Rev binding (F8-RRE), and the emission was blue-shifted. In addition, the local rotation of the tryptophan side chain was blocked in the protein-RRE complex. These results indicate that Trp45 directly interacts with the RRE. Rev is also shown to bind to 5S RNA, resulting in very similar changes in the time-resolved tryptophan fluorescence to those observed upon complexation of Rev with F8-RRE.

Amino Acid Sequence↗
PubMed1998

Recognition of sequence-directed DNA structure by the Klenow fragment of DNA polymerase I.

Time-resolved fluorescence spectroscopy was used to investigate the influence of sequence-directed DNA structure upon the interaction between the Klenow fragment of DNA polymerase I and a series of defined oligonucleotide primer/templates. 17/27-mer (primer/template) oligonucleotides containing a dansyl fluorophore conjugated to a modified deoxyuridine residue within the primer strand were used as substrates for binding to Klenow fragment. The time-resolved fluorescence anisotropy decay of the dansyl probe was analyzed in terms of two local environments, either solvent-exposed or buried, corresponding to primer/templates positioned with the primer 3' terminus in the polymerase site or the 3'-5' exonuclease site of the enzyme, respectively. Equilibrium constants for partitioning of DNA between the two sites were evaluated from the anisotropy decay data for primer/templates having different (A + T)-rich sequences flanking the primer 3' terminus. Primer/templates with AAAATG/TTTTAC and CGATAT/GCTATA terminal sequences (the nucleotides on the left refer to the last six bases at the 3' end of the primer, and the nucleotides on the right are the corresponding bases in the template) were bound mostly at the polymerase site. The introduction of single mismatches opposite the primer 3' terminus of these DNA substrates increased their partitioning into the 3'-5' exonuclease site, in accord with the results of an earlier study [Carver, T.E., Hochstrasser, R.A., and Millar, D.P. (1994) Proc. Natl. Acad. Sci. U.S.A. 91, 10670-10674]. In contrast, a primer/template with the terminal sequence CAATTT/GTTAAA, containing an A-tract element AATTT, exhibited a surprising preference for binding at the 3'-5' exonuclease site, despite the absence of mismatched bases in the DNA substrate. Interruption of the A-tract with a single AG step, to give the terminal sequence CAGTTT/GTCAAA, reversed the effect of the A-tract, causing the DNA to partition in favor of the polymerase site. Moreover, the presence of a single mismatch opposite the primer 3' terminus was also sufficient to reverse the effect of the A-tract, resulting in a distribution of DNA between polymerase and 3'-5' exonuclease sites that was similar to that observed for the other mismatched DNA substrates. Taken together, these results suggest that the A-tract adopts an unusual conformation that is disruptive to binding at the polymerase site. The effect of the A-tract on binding of DNA to the polymerase site is discussed in terms of the unusual helix structural parameters associated with these sequence elements and the difference between the local geometry of the A-tract and the conformation adopted by duplex DNA within the polymerase cleft. The results of this study show that in addition to base mismatches, Klenow fragment can also recognize irregularities in the helix geometry of perfectly base-paired DNA.

Base Composition↗
PubMed1998

Effects of mutations on the partitioning of DNA substrates between the polymerase and 3'-5' exonuclease sites of DNA polymerase I (Klenow fragment).

Site-directed mutagenesis and time-resolved fluorescence spectroscopy were used to evaluate the contributions of individual amino acid side chains to the binding of DNA primer-templates to the 3'-5' exonuclease site of the large proteolytic fragment (Klenow fragment) of DNA polymerase I. Mutations were introduced into side chains that have been shown crystallographically to be in close proximity to a DNA 3' terminus bound at the 3'-5' exonuclease site. The wild-type residues were replaced by alanine in each case. To assess the effects of the mutations on DNA binding, time-resolved fluorescence anisotropy measurements were performed on dansyl-labeled primer-templates bound to the mutant enzymes. In contrast to techniques that simply monitor the overall binding of proteins to DNA, the time-resolved fluorescence anisotropy technique was used to determine the fractional occupancies of the polymerase and 3'-5' exonuclease active sites of Klenow fragment. Equilibrium constants describing the partitioning of DNA between the two active sites were obtained for nine different mutant enzymes bound to both matched and mismatched DNA sequences. Mutations of Leu361 and Phe473 caused the largest effects, significantly destabilizing the binding of mismatched DNA substrates to the 3'-5' exonuclease site relative to DNA bound at the polymerase site, consistent with structural data showing that the side chains of these residues are involved in intimate hydrophobic interactions with the 3' terminal and penultimate bases of the primer strand [Beese, L., and Steitz, T. A. (1991) EMBO J. 10, 25-33]. Mutations of the His660 and Glu357 side chains also resulted in significant effects on the binding of mismatched DNA to the 3'-5' exonuclease site. Surprisingly, mutation of Tyr497 increased the partitioning of mismatched DNA into the 3'-5' exonuclease site, suggesting that the tyrosine side chain in the wildtype enzyme destabilizes substrate binding, despite crystallographic data showing that Tyr497 is H-bonded to the DNA substrate. The effects of mutating the amino acid side chains that serve as ligands to two divalent metal ions bound at the 3'-5' exonuclease site, designated A and B, indicated that metal A also helps to bind DNA to the 3'-5' exonuclease site. These results demonstrate that the time-resolved fluorescence anisotropy technique can be used to quantify the energetic contributions associated with each of the crystallographically defined DNA-protein contacts at the 3'-5' exonuclease site.

Amino Acid Substitution↗
PubMed1997

Crossover isomer bias is the primary sequence-dependent property of immobilized Holliday junctions.

Recombination of genes is essential to the evolution of genetic diversity, the segregation of chromosomes during cell division, and certain DNA repair processes. The Holliday junction, a four-arm, four-strand branched DNA crossover structure, is formed as a transient intermediate during genetic recombination and repair processes in the cell. The recognition and subsequent resolution of Holliday junctions into parental or recombined products appear to be critically dependent on their three-dimensional structure. Complementary NMR and time-resolved fluorescence resonance energy transfer experiments on immobilized four-arm DNA junctions reported here indicate that the Holliday junction cannot be viewed as a static structure but rather as an equilibrium mixture of two conformational isomers. Furthermore, the distribution between the two possible crossover isomers was found to depend on the sequence in a manner that was not anticipated on the basis of previous low-resolution experiments.

Base Sequence↗
PubMed1996

Conformational flexibility of three-way DNA junctions containing unpaired nucleotides.

Time-resolved fluorescence resonance energy transfer has been used to examine the global structure and conformational flexibility of three-way DNA junctions containing unpaired bases at the branch point. Three-way junctions were prepared with donor (fluorescein) and acceptor (tetramethyl-rhodamine) dyes attached to the ends of different helical arms in various pairwise combinations. The time-resolved fluorescence decay of the donor in each labeled junction was measured by time-correlated single photon counting. The distributions of donor-acceptor (D-A) distances present between each pair of labeled helices were recovered from analysis of the donor decay profiles using a Gaussian distribution model. The recovered D-A distance distributions reveal the mean distance between each pair of helices, as well as the range of distances that exists between each pair. For the junction lacking unpaired bases, the three mean interarm distances are similar, indicating an extended structure. In addition, a relatively broad range of distances is present between each pair of helices, showing that the structure is flexible. The addition of unpaired bases causes the junction to fold into a different structure, with one interarm distance being shorter than the other two. The change in overall geometry of the junction appears to be primarily due to the repositioning of one of the helices flanking the bulge. In bulged junctions containing unpaired thymine, cytosine, or adenine bases, the helix containing the 3' portion of the bulged strand appears to undergo the greatest change in its mean position relative to the other helices. In contrast, in the bulged junction containing unpaired guanine bases, the helix containing the 5' portion of the bulged strand is displaced. In all bulged junctions, there is a wide range of distances between the perturbed helix and the other two helices, indicating high mobility for the perturbed arm. These results indicate that the overall structure and conformational flexibility of three-way DNA junctions are sensitive to the presence of unpaired bases at the branch point of the junction and that the precise effect of a bulge depends on the nature of the unpaired bases.

Base Composition↗
PubMed1996

Time-resolved fluorescence spectroscopy.

Time-resolved fluorescence spectroscopy is used to monitor molecular interactions and motions that occur in the picosecond-nanosecond time range, and is especially useful in the analysis of biomolecular structure and dynamics. Recent advances in the application of time-resolved fluorescence spectroscopy to biological systems have led to a better understanding of the origin of nonexponential fluorescence decay in proteins, the use of tryptophan analogs as unique spectroscopic probes of protein-protein interactions, the detailed characterization of protein-folding processes and intermediates, and the development of new approaches to the study of DNA-protein interactions.

Animals↗
PubMed1996

Fluorescence studies of DNA and RNA structure and dynamics.

Fluorescence spectroscopy is increasingly being used as a technique for probing the structure and dynamics of nucleic acids. Recently, fluorescence methods have been used to elucidate the three-dimensional arrangement of complex DNA and RNA structures, characterize structural perturbations resulting from base bulges in helices and junctions, determine helical handedness in solution, and analyze the protein-induced melting of DNA.

DNA↗
PubMed1995

Interaction between the Escherichia coli Regulatory protein TyrR and DNA: a fluorescence footprinting study.

The Escherichia coli regulatory protein TyrR controls the expression of eight transcription units that encode proteins involved in the biosynthesis and transport of aromatic amino acids. It is a homodimer of 57 600 subunit molecular weight and has a binding site for ATP and weak ATPase activity. In the presence of ATP, TyrR binds tyrosine, which induces self-association of TyrR from a dimer to a hexamer. This report examines the interaction of TyrR with a 42 bp DNA oligonucleotide containing a centrally located binding site for TyrR (TyrR box). Replacement of a thymidine residue with an aminouridine residue at positions 7, 9, 13, 15, 19, 22, and 26 from one end of the 42mer enables labeling with fluorescein and successive placement of the label along the major groove of the DNA. The fluorescence footprinting of the oligonucleotide was followed using steady-state and time-resolved fluorescence methods. Binding of the TyrR dimer caused significant changes in the fluorescent properties of the labels attached to positions 13, 15, and 26, suggesting the involvement of these bases in the binding of the protein. Except for the position 15 conjugate, binding of the TyrR dimer caused little change in fluorescence intensity. Therefore, fluorescence anisotropy was used to follow the binding equilibrium. The fluorescence of the position 15 conjugate increased 1.6-fold on binding TyrR, suggesting that the fluorophore was in close contact with the protein. For all conjugates, the addition of tyrosine at the end of the titration with TyrR increased the anisotropy markedly, suggesting that the hexameric form of TyrR could bind the oligonucleotide. Two rotational correlation times were found for the labeled conjugates: one reflecting the motion of the probe at its point of attachment to the DNA (220-290 ps), the other reflecting the global tumbling of the labeled oligonucleotide (14-21 ns). On binding TyrR, changes in the correlation times and their associated amplitudes and changes in the range of angular motion of the probe depended on the position of the label. Evidence is presented that the binding of the TyrR hexamer, but not the TyrR dimer, affects regions that flank the binding sequence. The results support the hypothesis that the binding of the TyrR hexamer is responsible for interaction between tandem TyrR boxes in the tyrR regulon.

Base Sequence↗
PubMed1995

The nucleotide analog 2-aminopurine as a spectroscopic probe of nucleotide incorporation by the Klenow fragment of Escherichia coli polymerase I and bacteriophage T4 DNA polymerase.

The fluorescent properties and their sensitivity to the surrounding environment of the nucleotide analog 2-aminopurine (2-AP) have been well documented. In this paper we describe the use of 2-AP as a direct spectroscopic probe of the mechanism of nucleotide incorporation by Escherichia coli Pol I Klenow fragment (KF) and bacteriophage T4 DNA polymerase. The nucleotidyl transfer reaction may be monitored in real time by following the fluorescence of 2-AP, allowing the detection of transient intermediates along the reaction pathway that are inaccessible through traditional radioactive assays. Previous studies with Klenow fragment [Kuchta, R. D., Mizrahi, V., Benkovic, P. A., Johnson, K. A., & Benkovic, S. J. (1987) Biochemistry 26, 8410-8417] have revealed the presence of a nonchemical step prior to chemistry and have identified this conformational change as the rate-limiting step of correct nucleotide incorporation. During correct incorporation, phosphodiester bond formation occurs at a rate greater than the conformational change and has not been measured. However, during misinsertion, the rate of the chemical step becomes partially rate limiting and it becomes possible to detect both steps. We have successfully decoupled the chemical and conformational change steps for nucleotide insertion by KF using the misincorporation reaction, and we present direct spectroscopic evidence for an activated KF'-DNA-dNTP species following the conformational change step which features hydrogen bonding between the incoming and template bases. In addition, we have utilized these same experiments to demonstrate the existence of a similar nonchemical step in the mechanism of dNTP incorporation by bacteriophage T4 DNA polymerase.(ABSTRACT TRUNCATED AT 250 WORDS)

2-Aminopurine↗
PubMed1994

Direct measurement of thermodynamic and kinetic parameters of DNA triple helix formation by fluorescence spectroscopy.

Direct measurement of thermodynamic and kinetic parameters of oligonucleotide-directed DNA triple helix formation has been achieved by fluorescence spectroscopic methods. Fluorescence resonance energy transfer (FRET) was used to study the binding of an acceptor-labeled single-stranded oligonucleotide to a donor-labeled DNA duplex. Equilibrium binding constants and association rate constants for triplex formation between 5'-tetramethylrhodamine-labeled 11-mer, 13-mer, and 15-mer homopyrimidine oligonucleotides and a 5'-fluorescein-labeled, 25-bp DNA duplex containing a 15-bp homopurine site were determined by FRET measurements, and the values were in close agreement with those determined by established methods. The thermal dissociation profile of the triplex-to-duplex transition was also directly observed by FRET and was consistent with the triplex melting curves obtained by UV absorbance measurements. In addition, a homogeneous fluorescence anisotropy assay is described which enables determination of the binding constants between 5'-tetramethylrhodamine-labeled 11-mer and 13-mer homopyrimidine oligonucleotides and unlabeled 25-, 30-, and 50-bp double-stranded DNA containing a homopurine target site. These results demonstrate the utility of nonradioactive fluorescence measurements as an efficient method for studying triple helix formation under homogeneous solution conditions and highlight the uniqueness of the FRET method for obtaining equilibrium, kinetic, and thermal dissociation data in a straightforward manner.

Base Sequence↗
PubMed1994

Proofreading DNA: recognition of aberrant DNA termini by the Klenow fragment of DNA polymerase I.

Fluorescence depolarization decays were measured for 5-dimethylaminonaphthalene-1-sulfonyl (dansyl) probes attached internally to 17-mer.27-mer oligonucleotides bound to Klenow fragment of DNA polymerase I. The time-resolved motions of the dansyl probes were sensitive indicators of DNA-protein contacts, showing that the protein binds to DNA with two footprints, corresponding to primer termini at either the polymerase or 3'-5' exonuclease sites. We examined complexes of Klenow fragment with DNAs containing various base mismatches. Single mismatches at the primer terminus caused a 3- to 4-fold increase in the equilibrium partitioning of DNA into the exonuclease site; the largest effects were observed for purine-purine mismatches. Two or more consecutive G.G mismatches caused the DNA to bind exclusively at the exonuclease site, with a partitioning constant at least 250-fold greater than that of the corresponding matched DNA sequence. Internal single mismatches produced larger effects than the same mismatch at the primer terminus, with a delta delta G relative to the matched sequence of -1.1 to -1.3 kcal/mol for mismatches located 2, 3, or 4 bases from the primer terminus. Although part of the observed effects may be attributed to the increased melting capacity of the DNA, it appears that the polymerase site also promotes movement of DNA into the exonuclease site by rejecting aberrant primer termini. These effects suggest that the polymerase and exonuclease sites act together to recognize specific errors that distort the primer terminus, such as frameshifts, in addition to proofreading misincorporated bases.

Base Sequence↗
PubMed1994

Melting of a DNA helix terminus within the active site of a DNA polymerase.

Accurate synthesis of DNA by polymerase is due in part to the selective removal of misincorporated nucleotides by a 3'-5' exonuclease activity (proofreading). Proofreading by an exonuclease domain containing a single-stranded DNA binding site may involve local melting of a duplex DNA substrate. Here we use time-resolved fluorescence spectroscopy to analyze the local melting of a DNA duplex terminus induced by the Klenow fragment of DNA polymerase I. Four oligodeoxynucleotide primer/templates were prepared, each containing the fluorescent adenine analog 2-aminopurine (A*) at the primer 3' terminus, and one of the common DNA bases opposite the A* residue. Fluorescence decays of the duplex DNAs and the single primer oligonucleotide were jointly analyzed using global analysis procedures. Four lifetime components were resolved in the duplex DNAs, representing distinct conformational states of the terminal A* residue: paired A* bases, partially stacked A* bases, and extended A* bases. The variation of the apparent fraction of paired A* bases with temperature was in accord with optical melting data, and the extent of base pairing observed in each duplex was consistent with the base-pairing preferences of A* established in other studies. These results establish that the fluorescence decay characteristics of A* can be used to examine base-pairing interactions at a DNA duplex terminus. Since the fluorescence of A* can be observed without interference from protein amino acid residues, unlike existing methods for monitoring DNA melting transitions, this method was used to examine the extent to which Klenow fragment could induce fraying at each duplex terminus.(ABSTRACT TRUNCATED AT 250 WORDS)

2-Aminopurine↗
PubMed1994

Real time kinetics of restriction endonuclease cleavage monitored by fluorescence resonance energy transfer.

The kinetics of PaeR7 endonuclease-catalysed cleavage reactions of fluorophor-labeled oligonucleotide substrates have been examined using fluorescence resonance energy transfer (FRET). A series of duplex substrates were synthesized with an internal CTCGAG PaeR7 recognition site and donor (fluorescein) and acceptor (rhodamine) dyes conjugated to the opposing 5' termini. The time-dependent increase in donor fluorescence resulting from restriction cleavage of these substrates was continuously monitored and the initial rate data was fitted to the Michaelis-Menten equation. The steady state kinetic parameters for these substrates were in agreement with the rate constants obtained from a gel electrophoresis-based fixed time point assay using radiolabeled substrates. The FRET method provides a rapid continuous assay as well as high sensitivity and reproducibility. These features should make the technique useful for the study of DNA-cleaving enzymes.

Base Sequence↗
PubMed1994

A fluorescence-based assay for monitoring helicase activity.

A continuous fluorescence-based assay is described for measuring helicase-mediated unwinding of duplex DNA. The assay utilizes an oligonucleotide substrate containing the fluorescent adenine analog, 2-aminopurine, at regular intervals. 2-Aminopurine forms a Watson-Crick-type base pair with thymine and does not distort normal B-form DNA. Fluorescence of the 2-aminopurines within this oligonucleotide is quenched 2-fold upon its hybridization to a complementary strand. Unwinding of this substrate by the T4 dda helicase restores the fluorescence of the 2-aminopurines and is easily followed using stopped-flow or steady-state fluorescence spectroscopy. The flourescence-based assay provides rate data comparable to that obtained from conventional discontinuous assays using labeled substrates and additionally furnishes a means for following a single turnover. This assay should prove useful for defining the mechanism by which helicases unwind duplex DNA.

2-Aminopurine↗
PubMed1993

Conformational distributions of a four-way DNA junction revealed by time-resolved fluorescence resonance energy transfer.

Conformational distributions of a four-way DNA junction have been examined by time-resolved fluorescence resonance energy transfer (FRET). A series of dye-labeled junctions were synthesized with donor (fluorescein) and acceptor (tetramethylrhodamine) dyes conjugated to the 5' termini of the duplex arms in all six pairwise combinations. The fluorescence decay of the donor in each junction was measured by time-correlated single-photon counting. The distributions of donor-acceptor (D-A) distances present between each pair of arms were recovered from the donor decays using a continuous Gaussian distribution model. The overall geometry of the four-way junction defined by the six mean D-A distances was consistent with a stacked-X structure, wherein pairs of duplex arms associate to form two continuous domains. Large differences were observed in the widths of the D-A distance distributions, depending on which pair of arms were labeled with the donor and acceptor dyes. Distances measured along the stacking domains were characterized by relatively narrow distributions, indicating that these domains were rigid, whereas distances between stacking domains had broader distributions, reflecting variability in the angle between the two domains. The distances described by broad distributions were overestimated by steady-state FRET measurements. These results suggest that an ensemble of stacked-X structures are present in solution, characterized by differences in the small angle between the stacking domains. Temperature and solvent effects on the recovered distribution widths provide an indication of flexibility in the four-way junction.

Base Sequence↗
PubMed1992

Distance distribution in a dye-linked oligonucleotide determined by time-resolved fluorescence energy transfer.

Fluorescence energy transfer is potentially a useful technique for obtaining structural and dynamic information on duplex and branched DNA molecules suitably labeled with donor and acceptor dyes. We have assessed the accuracy and limitations of FET measurements in nucleic acids with respect to the localization of the dyes and the flexibility of the dye-DNA linkages. A nine base-pair duplex oligonucleotide was synthesized with donor and acceptor dyes linked at the opposing 5' termini by alkyl chains. A careful analysis of the fluorescence decay of the donor revealed that the donor-acceptor distance in this molecule was not well defined, but was described by a rather broad distribution. The mean donor-acceptor distance and the distribution of distances have been recovered from the donor decay. Orientational effects on energy transfer have been included in the analysis. The implications of these findings for FET measurements in nucleic acids are considered.

Base Sequence↗
PubMed1991

Interaction of DNA with the Klenow fragment of DNA polymerase I studied by time-resolved fluorescence spectroscopy.

The interaction of a fluorescent duplex DNA oligomer with the Klenow fragment of DNA polymerase I from Escherichia coli has been studied in solution by using time-resolved fluorescence spectroscopy. An aminonaphthalenesulfonate (dansyl) fluorescent probe was linked by a propyl chain to a C5-modified uridine base located at a specific site in the primer strand of the DNA oligomer. The fluorescent oligomer bound tightly to the Klenow fragment (KD = 7.9 nM), and the probe's position within the DNA-protein complex was varied by stepwise elongation of the primer strand upon addition of the appropriate deoxynucleoside triphosphates. The decay of the total fluorescence intensity and the polarization anisotropy were measured with a picosecond laser and a time-correlated single photon counting system. The fluorescence lifetimes, the correlation time for internal rotation, and the angular range of internal rotation varied according to the probe's position within the DNA-protein complex. These results showed that five or six bases of the primer strand upstream of the 3' terminus were in contact with the protein and that within this contact region there were differences in the degree of solvent accessibility and the closeness of contact. Further, a minor binding mode of the DNA-protein complex was identified, on the basis of heterogeneity of the probe environment observed when the probe was positioned seven bases upstream from the primer 3' terminus, which resulted in a distinctive "dip and rise" in the anisotropy decay. Experiments with an epoxy-terminated DNA oligomer and a site-directed mutant protein established that the labeled DNA was binding at the polymerase active site (major form) and at the spatially distinct 3'----5' exonuclease active site (minor form). The abundance of each of these distinct binding modes of the DNA-protein complex was estimated under solution conditions by analyzing the anisotropy decay of the dansyl probe. About 12% of the labeled DNA was bound at the 3'----5' exonuclease site. This method should be useful for investigating the editing mechanism of this important enzyme.

Base Sequence↗