Four-color DNA sequencing by synthesis on a chip using photocleavable fluorescent nucleotides

We report four-color DNA sequencing by synthesis (SBS) on a chip, using four photocleavable fluorescent nucleotide analogues (dGTP-PC-Bodipy-FL-510, dUTP-PC-R6G, dATP-PC-ROX, and dCTP-PC-Bodipy-650) (PC, photocleavable; Bodipy, 4,4-difluoro-4-bora-3alpha,4alpha-diaza-s-indacene; ROX, 6-carboxy-X-rhodamine; R6G, 6-carboxyrhodamine-6G). Each nucleotide analogue consists of a different fluorophore attached to the 5 position of the pyrimidines and the 7 position of the purines through a photocleavable 2-nitrobenzyl linker. After verifying that these nucleotides could be successfully incorporated into a growing DNA strand in a solution-phase polymerase reaction and the fluorophore could be cleaved using laser irradiation ( approximately 355 nm) in 10 sec, we then performed an SBS reaction on a chip that contains a self-priming DNA template covalently immobilized by using 1,3-dipolar azide-alkyne cycloaddition. The DNA template was produced by PCR, using an azido-labeled primer, and the self-priming moiety was attached to the immobilized DNA template by enzymatic ligation. Each cycle of SBS consists of the incorporation of the photocleavable fluorescent nucleotide into the DNA, detection of the fluorescent signal, and photocleavage of the fluorophore. The entire process was repeated to identify 12 continuous bases in the DNA template. These results demonstrate that photocleavable fluorescent nucleotide analogues can be incorporated accurately into a growing DNA strand during a polymerase reaction in solution and on a chip. Moreover, all four fluorophores can be detected and then efficiently cleaved using near-UV irradiation, thereby allowing continuous identification of the DNA template sequence. Optimization of the steps involved in this SBS approach will further increase the read-length.     

Four-color DNA sequencing by synthesis using cleavable fluorescent nucleotide reversible terminators

DNA sequencing by synthesis (SBS) on a solid surface during polymerase reaction offers a paradigm to decipher DNA sequences. We report here the construction of such a DNA sequencing system using molecular engineering approaches. In this approach, four nucleotides (A, C, G, T) are modified as reversible terminators by attaching a cleavable fluorophore to the base and capping the 3'-OH group with a small chemically reversible moiety so that they are still recognized by DNA polymerase as substrates. We found that an allyl moiety can be used successfully as a linker to tether a fluorophore to 3'-O-allyl-modified nucleotides, forming chemically cleavable fluorescent nucleotide reversible terminators, 3'-O-allyl-dNTPs-allyl-fluorophore, for application in SBS. The fluorophore and the 3'-O-allyl group on a DNA extension product, which is generated by incorporating 3'-O-allyl-dNTPs-allyl-fluorophore in a polymerase reaction, are removed simultaneously in 30 s by Pd-catalyzed deallylation in aqueous buffer solution. This one-step dual-deallylation reaction thus allows the reinitiation of the polymerase reaction and increases the SBS efficiency. DNA templates consisting of homopolymer regions were accurately sequenced by using this class of fluorescent nucleotide analogues on a DNA chip and a four-color fluorescent scanner.     

3'-O-modified nucleotides as reversible terminators for pyrosequencing

Pyrosequencing is a method used to sequence DNA by detecting the pyrophosphate (PPi) group that is generated when a nucleotide is incorporated into the growing DNA strand in polymerase reaction. However, this method has an inherent difficulty in accurately deciphering the homopolymeric regions of the DNA templates. We report here the development of a method to solve this problem by using nucleotide reversible terminators. These nucleotide analogues are modified with a reversible chemical moiety capping the 3'-OH group to temporarily terminate the polymerase reaction. In this way, only one nucleotide is incorporated into the growing DNA strand even in homopolymeric regions. After detection of the PPi for sequence determination, the 3'-OH of the primer extension products is regenerated through different deprotection methods. Using an allyl or a 2-nitrobenzyl group as the reversible moiety to cap the 3'-OH of the four nucleotides, we have synthesized two sets of 3'-O-modified nucleotides, 3'-O-allyl-dNTPs and 3'-O-(2-nitrobenzyl)-dNTPs as reversible terminators for pyrosequencing. The capping moiety on the 3'-OH of the DNA extension product is efficiently removed after PPi detection by either a chemical method or photolysis. To sequence DNA, templates containing homopolymeric regions are immobilized on Sepharose beads, and then extension-signal detection-deprotection cycles are conducted by using the nucleotide reversible terminators on the DNA beads to unambiguously decipher the sequence of DNA templates. Our results establish that this reversible-terminator-pyrosequencing approach can be potentially developed into a powerful methodology to accurately determine DNA sequences.     

Four-color DNA sequencing with 3'-O-modified nucleotide reversible terminators and chemically cleavable fluorescent dideoxynucleotides

DNA sequencing by synthesis (SBS) on a solid surface during polymerase reaction can decipher many sequences in parallel. We report here a DNA sequencing method that is a hybrid between the Sanger dideoxynucleotide terminating reaction and SBS. In this approach, four nucleotides, modified as reversible terminators by capping the 3'-OH with a small reversible moiety so that they are still recognized by DNA polymerase as substrates, are combined with four cleavable fluorescent dideoxynucleotides to perform SBS. The ratio of the two sets of nucleotides is adjusted as the extension cycles proceed. Sequences are determined by the unique fluorescence emission of each fluorophore on the DNA products terminated by ddNTPs. On removing the 3'-OH capping group from the DNA products generated by incorporating the 3'-O-modified dNTPs and the fluorophore from the DNA products terminated with the ddNTPs, the polymerase reaction reinitiates to continue the sequence determination. By using an azidomethyl group as a chemically reversible capping moiety in the 3'-O-modified dNTPs, and an azido-based cleavable linker to attach the fluorophores to the ddNTPs, we synthesized four 3'-O-azidomethyl-dNTPs and four ddNTP-azidolinker-fluorophores for the hybrid SBS. After sequence determination by fluorescence imaging, the 3'-O-azidomethyl group and the fluorophore attached to the DNA extension product via the azidolinker are efficiently removed by using Tris(2-carboxyethyl)phosphine in aqueous solution that is compatible with DNA. Various DNA templates, including those with homopolymer regions, were accurately sequenced with a read length of >30 bases by using this hybrid SBS method on a chip and a four-color fluorescence scanner.     

Photocleavable fluorescent nucleotides for DNA sequencing on a chip constructed by site-specific coupling chemistry

DNA sequencing by synthesis on a solid surface offers new paradigms to overcome limitations of electrophoresis-based sequencing methods. Here we report DNA sequencing by synthesis using photocleavable (PC) fluorescent nucleotides [dUTP-PC-4,4-difluoro-4-bora-3 alpha,4 alpha-diaza-s-indacene (Bodipy)-FL-510, dCTP-PC-Bodipy-650, and dUTP-PC-6-carboxy-X-rhodamine (ROX)] on a glass chip constructed by 1,3-dipolar azide-alkyne cycloaddition coupling chemistry. Each nucleotide analogue consists of a different fluorophore attached to the base through a PC 2-nitrobenzyl linker. We constructed a DNA microarray by using the 1,3-dipolar cycloaddition chemistry to site-specifically attach azido-modified DNA onto an alkyne-functionalized glass chip at room temperature under aqueous conditions. After verifying that the polymerase reaction could be carried out successfully on the above-described DNA array, we then performed a sequencing reaction on the chip by using a self-primed DNA template. In the first step, we extended the primer using DNA polymerase and dUTP-PC-Bodipy-FL-510, detected the fluorescent signal from the fluorophore Bodipy-FL-510, and then cleaved the fluorophore using 340 nm UV irradiation. This process was followed by extension of the primer with dCTP-PC-Bodipy-650 and the subsequent detection of the fluorescent signal from Bodipy-650 and its photocleavage. The same procedure was also performed by using dUTP-PC-ROX. The entire process was repeated five times by using the three fluorescent nucleotides to identify 7 bases in the DNA template. These results demonstrate that the PC nucleotide analogues can be incorporated accurately into a growing DNA strand during polymerase reaction on a chip, and the fluorophore can be detected and then efficiently cleaved using near-UV irradiation, thereby allowing the continuous identification of the template sequence.     

A photocleavable fluorescent nucleotide for DNA sequencing and analysis

DNA sequencing by synthesis during a polymerase reaction using laser-induced fluorescence detection is an approach that has a great potential to increase the throughput and data quality of DNA sequencing. We report the design and synthesis of a photocleavable fluorescent nucleoside triphosphate, one of the essential molecules required for the sequencing-by-synthesis approach. We synthesized this nucleoside triphosphate by attaching a fluorophore, 4,4-difluoro-5,7-dimethyl-4-bora-3alpha,4alpha-diaza-s-indacene propionic acid (BODIPY), to the 5 position of 2'-deoxyuridine triphosphate via a photocleavable 2-nitrobenzyl linker. We demonstrate that the nucleotide analogue can be faithfully incorporated by a DNA polymerase Thermo Sequenase into the growing DNA strand in a DNA-sequencing reaction and that its incorporation does not hinder the addition of the subsequent nucleotide. These results indicate that the nucleotide analogue is an excellent substrate for Thermo Sequenase. We also systematically studied the photocleavage of the fluorescent dye from a DNA molecule that contained the nucleotide analogue. UV irradiation at 340 nm of the DNA molecule led to the efficient release of the fluorescent dye, ensuring that a previous fluorescence signal did not leave any residue that could interfere with the detection of the next nucleotide. Thus, our results indicate that it should be feasible to use four different fluorescent dyes with distinct fluorescence emissions as unique tags to label the four nucleotides (A, C, G, and T) through the photocleavable 2-nitrobenzyl linker. These fluorescent tags can be removed easily by photocleavage after the identification of each nucleotide in the DNA sequencing-by-synthesis approach.     

Photocleavage of a 2-nitrobenzyl linker bridging a fluorophore to the 5' end of DNA

Three single-stranded DNA molecules of different lengths were synthesized and characterized, each containing a fluorescent dye (6-carboxyfluorescein) connected to the 5' end via a photocleavable 2-nitrobenzyl linker and a biotin moiety at the 3' end. UV irradiation (lambda approximately 340 nm) of solutions containing these fluorescent DNA molecules caused the complete cleavage of the nitrobenzyl linker, separating the fluorophore from the DNA. The photocleavage products were characterized by HPLC and matrix-assisted laser desorption ionization/time-of-flight mass spectrometry. Our experimental results indicated that the proximity of the chromophore 6-carboxyfluorescein to the 2-nitrobenzyl linker did not hinder the quantitative photocleavage of the linker in the DNA molecules. The biotin moiety allowed immobilization of the fluorescent DNA on streptavidin-coated glass chips. The photocleavage of the immobilized DNA was investigated directly by fluorescence spectroscopy. The results demonstrated that close to 80% of the fluorophore was removed from the immobilized DNA after UV irradiation at 340 nm. These results strongly support the application of the 2-nitrobenzyl moiety as an efficient photocleavable linker, connecting fluorescent probes to DNA molecules for a variety of biological analyses such as DNA sequencing by synthesis.     

A structural rationale for stalling of a replicative DNA polymerase at the most common oxidative thymine lesion, thymine glycol

Thymine glycol (Tg) is a common product of oxidation and ionizing radiation, including that used for cancer treatment. Although Tg is a poor mutagenic lesion, it has been shown to present a strong block to both repair and replicative DNA polymerases. The 2.65-A crystal structure of a binary complex of the replicative RB69 DNA polymerase with DNA shows that the templating Tg is intrahelical and forms a regular Watson-Crick base pair with the incorporated A. The C5 methyl group protrudes axially from the ring of the damaged pyrimidine and hinders stacking of the adjacent 5' template guanine. The position of the displaced 5' template guanine is such that the next incoming nucleotide cannot be incorporated into the growing primer strand, and it explains why primer extension past the lesion is prohibited even though DNA polymerases can readily incorporate an A across from the Tg lesion.     

Mutagenesis during in vitro DNA synthesis.

The error frequency of in vitro DNA synthesis using a natural DNA template has been measured with a biological assay for nucleotide substitutions. phiX174 DNA containing an amber mutation was copied in vitro by Escherichia coli DNA polymerase I, and the reversion frequency of the progeny DNA was determined by transfection of E. coli spheroplasts. E. coli polymerase I makes less than 1 mistake at the am3 locus for every 7700 nucleotides incorporated under standard reaction conditions. Substitution of Mn2+ for Mg2+ and unequal concentrations of deoxynucleoside triphosphate substrates raises this mutation frequency to greater than 1 in 1000. Thus, E. coli DNA polymerase I can copy natural DNA templates with high fidelity and its accuracy can be affected by alterations in reaction conditions.     

Sequential structures provide insights into the fidelity of RNA replication

RNA virus replication is an error-prone event caused by the low fidelity of viral RNA-dependent RNA polymerases. Replication fidelity can be decreased further by the use of mutagenic ribonucleoside analogs to a point where viral genetic information can no longer be maintained. For foot-and-mouth disease virus, the antiviral analogs ribavirin and 5-fluorouracil have been shown to be mutagenic, contributing to virus extinction through lethal mutagenesis. Here, we report the x-ray structure of four elongation complexes of foot-and-mouth disease virus polymerase 3D obtained in presence of natural substrates, ATP and UTP, or mutagenic nucleotides, ribavirin triphosphate and 5-fluorouridine triphosphate with different RNAs as template-primer molecules. The ability of these complexes to synthesize RNA in crystals allowed us to capture different successive replication events and to define the critical amino acids involved in (i) the recognition and positioning of the incoming nucleotide or analog; (ii) the positioning of the acceptor base of the template strand; and (iii) the positioning of the 3'-OH group of the primer nucleotide during RNA replication. The structures identify key interactions involved in viral RNA replication and provide insights into the molecular basis of the low fidelity of viral RNA polymerases.     

THE ENZYMATIC TERMINATION OF POLYDEOXYNUCLEOTIDES BY 2′,3′-DIDEOXYADENOSINE TRIPHOSPHATE

The nucleoside 2',3'-dideoxyadenosine is lethal to E. coli and blocks DNA synthesis irreversibly. The hypothesis that a derived dideoxynucleoside triphosphate is incorporated terminally into the cellular DNA has been tested in an in vitro system. The triphosphate of dideoxyadenosine was synthesized and shown to inhibit the in vitro synthesis of DNA by purified E. coli DNA polymerase. The kinetics of inhibition of nucleotide incorporation and pyrophosphate exchange were studied. Early in synthesis the dideoxynucleotide is a competitive inhibitor of the enzyme. Subsequently, synthesis is almost completely arrested.Radioactive dideoxyadenosine triphosphate was prepared. The compound was shown to be incorporated enzymatically into dAT copolymer to the extent of about one molecule per molecule of template. It is released from such templates by DNA polymerase at less than 1 per cent of the rate of release of other natural nucleotides. The label is released by snake venom phosphodiesterase far more rapidly than total nucleotides. This nucleoside triphosphate has thus been shown to be a competitive inhibitor of DNA polymerase and a terminator of polydeoxynucleotide chains.     

5-Azido-2''-deoxyuridine 5''-triphosphate: a photoaffinity-labeling reagent and tool for the enzymatic synthesis of photoactive DNA.

We have synthesized the photoactive deoxyuridine nucleotide 5-azido-2'-deoxyuridine 5'-triphosphate (5-N3dUTP) and used it to synthesize light-sensitive DNA by enzymatic incorporation. In the absence of ultraviolet light, 5-N3dUTP is a substrate for Escherichia coli DNA polymerase I. In in vitro DNA synthesis reactions using bacteriophage M13 single-stranded DNA as the template and 5-N3dUTP in place of dTTP, a photoactive complementary strand was synthesized by DNA polymerase I. The complementary strand was not synthesized when the 5-N3dUTP was substituted for dCTP or when it was exposed to ultraviolet light prior to the addition of DNA polymerase I. Using a synthetic lac operator template of 26 bases and a 15-base primer, we generated a photoactive 26-base-pair lac operator by enzymatically incorporating 5-N3dUMP with DNA polymerase I. Crosslinking of this photoactive DNA fragment to lac repressor was totally dependent on the presence of UV light and was reduced 78% by 150 microM isopropyl beta-D-thiogalactoside. Under the same conditions no crosslinking to lac repressor was observed using a nonphotoactive 26-base-pair lac operator. Photoactivatable deoxyuridine analogs have potential application as reagents to crosslink DNA binding proteins to 5-azidouracil-containing DNA and as active-site-directed photoaffinity labelling reagents.     

Single-molecule measurements of synthesis by DNA polymerase with base-pair resolution

The catalytic mechanism of DNA polymerases involves multiple steps that precede and follow the transfer of a nucleotide to the 3′-hydroxyl of the growing DNA chain. Here we report a single-molecule approach to monitor the movement of E. coli DNA polymerase I (Klenow fragment) on a DNA template during DNA synthesis with single base-pair resolution. As each nucleotide is incorporated, the single-molecule Förster resonance energy transfer intensity drops in discrete steps to values consistent with single-nucleotide incorporations. Purines and pyrimidines are incorporated with comparable rates. A mismatched primer/template junction exhibits dynamics consistent with the primer moving into the exonuclease domain, which was used to determine the fraction of primer-termini bound to the exonuclease and polymerase sites. Most interestingly, we observe a structural change after the incorporation of a correctly paired nucleotide, consistent with transient movement of the polymerase past the preinsertion site or a conformational change in the polymerase. This may represent a previously unobserved step in the mechanism of DNA synthesis that could be part of the proofreading process.     

Orchestration of cooperative events in DNA synthesis and repair mechanism unraveled by transition path sampling of DNA polymerase beta's closing

Our application of transition path sampling to a complex biomolecular system in explicit solvent, the closing transition of DNA polymerase beta, unravels atomic and energetic details of the conformational change that precedes the chemical reaction of nucleotide incorporation. The computed reaction profile offers detailed mechanistic insights into, as well as kinetic information on, the complex process essential for DNA synthesis and repair. The five identified transition states extend available experimental and modeling data by revealing highly cooperative dynamics and critical roles of key residues (Arg-258, Phe-272, Asp-192, and Tyr-271) in the enzyme's function. The collective cascade of these sequential conformational changes brings the DNA/DNA polymerase beta system to a state nearly competent for the chemical reaction and suggests how subtle residue motions and conformational rate-limiting steps affect reaction efficiency and fidelity; this complex system of checks and balances directs the system to the chemical reaction and likely helps the enzyme discriminate the correct from the incorrect incoming nucleotide. Together with the chemical reaction, these conformational features may be central to the dual nature of polymerases, requiring specificity (for correct nucleotide selection) as well as versatility (to accommodate different templates at every step) to maintain overall fidelity. Besides leading to these biological findings, our developed protocols open the door to other applications of transition path sampling to long-time, large-scale biomolecular reactions.     

From the Cover: PNAS Plus: Design and characterization of a nanopore-coupled polymerase for single-molecule DNA sequencing by synthesis on an electrode array

Scalable, high-throughput DNA sequencing is a prerequisite for precision medicine and biomedical research. Recently, we presented a nanopore-based sequencing-by-synthesis (Nanopore-SBS) approach, which used a set of nucleotides with polymer tags that allow discrimination of the nucleotides in a biological nanopore. Here, we designed and covalently coupled a DNA polymerase to an α-hemolysin (αHL) heptamer using the SpyCatcher/SpyTag conjugation approach. These porin–polymerase conjugates were inserted into lipid bilayers on a complementary metal oxide semiconductor (CMOS)-based electrode array for high-throughput electrical recording of DNA synthesis. The designed nanopore construct successfully detected the capture of tagged nucleotides complementary to a DNA base on a provided template. We measured over 200 tagged-nucleotide signals for each of the four bases and developed a classification method to uniquely distinguish them from each other and background signals. The probability of falsely identifying a background event as a true capture event was less than 1.2%. In the presence of all four tagged nucleotides, we observed sequential additions in real time during polymerase-catalyzed DNA synthesis. Single-polymerase coupling to a nanopore, in combination with the Nanopore-SBS approach, can provide the foundation for a low-cost, single-molecule, electronic DNA-sequencing platform.DNA sequencing is a fundamental technology in the biological and medical sciences (1). Advances in sequencing technology have enabled the growth of interest in individualized medicine with the hope of better treating human disease. The cost of genome sequencing has dropped by five orders of magnitude over the last decade but still remains out of reach as a conventional clinical tool (2, 3). Thus, the development of new, high-throughput, accurate, low-cost DNA-sequencing technologies is a high priority. Ensemble sequencing-by-synthesis (SBS) platforms dominate the current landscape. During SBS, a DNA polymerase binds and incorporates a nucleotide analog complementary to the template strand. Depending on the instrumentation, this nucleotide is identified either by its associated label or the appearance of a chemical by-product upon incorporation (4). These platforms take advantage of a high-fidelity polymerase reaction but require amplification and have limited read lengths (5). Recently, single-molecule strategies have been shown to have great potential to achieve long read lengths, which is critical for highly scalable and reliable genomic analysis (6–9). Pacific Biosciences’ SMRT SBS approach has been used for this purpose but has lower throughput and higher cost compared with current second-generation technology (10).Since the first demonstration of single-molecule characterization by a biological nanopore two decades ago (11), interest has grown in using nanopores as sensors for DNA base discrimination. One approach is strand sequencing, in which each base is identified as it moves through an ion-conducting channel, ideally producing a characteristic current blockade event for each base. Progress in nanopore sequencing has been hampered by two physical limitations. First, single-base translocation can be too rapid for detection (1–3 μs per base), and second, structural similarities between bases make them difficult to identify unambiguously (12). Some attempts to address these issues have used enzymes as molecular motors to control single-stranded DNA (ssDNA) translocation speeds but still rely on identifying multiple bases simultaneously (13–15). Other approaches used exonuclease to cleave a single nucleoside-5′-monophosphate that then passes through the pore (16), or modified the pore opening with a cyclodextrin molecule to slow translocation and increase resolution for individual base detection (17, 18). All of these techniques rely on detecting similarly sized natural bases, which produce relatively similar current blockade signatures. Additionally, no strategies for covalently linking a single enzyme to a multimeric nanopore have been published.Recently, we reported a method for SBS with nanopore detection (19, 20). This approach has two distinct features: the use of nucleotides with specific tags to enhance base discrimination and a ternary DNA polymerase complex to hold the tagged nucleotides long enough for tag recognition by the nanopore. As shown in Fig. 1, a single DNA polymerase is coupled to a membrane-embedded nanopore by a short linker. Next, template and four uniquely tagged nucleotides are added to initiate DNA synthesis. During formation of the ternary complex, a polymerase binds to a complementary tagged nucleotide; the tag specific for that nucleotide is then captured in the pore. Each tag is designed to have a different size, mass, or charge, so that they generate characteristic current blockade signatures, uniquely identifying the added base. This system requires a single polymerase coupled to each nanopore to ensure any signal represents sequencing information from only one DNA template at a time. Kumar et al. (19) demonstrated that nucleotides tagged with four different polyethylene glycol (PEG) molecules at the terminal phosphate were good substrates for polymerase and that the tags could generate distinct signals as they translocate through the nanopore. These modifications enlarge the discrimination of the bases by the nanopore relative to the use of the natural nucleotides. We recently expanded upon this work by replacing the four PEG polymers with oligonucleotide-based tags and showed that a DNA polymerase coupled to the nanopore could sequentially add these tagged nucleotides to a growing DNA strand to perform Nanopore-SBS (20). Although this work showcased the promise of this technology, it did not describe in detail how to build a protein construct capable of Nanopore-SBS and did not obtain enough data to develop a statistical framework to uniquely distinguish the tagged nucleotides from each other.Open in a separate windowFig. 1.Principle of single-molecule DNA sequencing by a nanopore using tagged nucleotides. Each of the four nucleotides carries a different polymer tag (green square, A; red oval, T; blue triangle, C; black square, G). During SBS, the complementary nucleotide (T shown here) forms a tight complex with primer/template DNA and the nanopore-coupled polymerase. As the tagged nucleotides are incorporated into the growing DNA template, their tags, attached via the 5′-phosphate, are captured in the pore lumen, which results in a unique current blockade signature (Bottom). At the end of the polymerase catalytic reaction, the tag is released, ending the current blockade, which returns to open-channel reading at this time. For the purpose of illustration, four distinct tag signatures are shown in the order of their sequential capture. A large array of such nanopores could lead to highly parallel, high-throughput DNA sequencing.Here, we describe the design and characterization of a protein construct capable of carrying out Nanopore-SBS (Fig. 1). A porin attached to a single DNA polymerase molecule is inserted into a lipid bilayer formed on an electrode array. The polymerase synthesizes a new DNA strand using four uniquely tagged nucleotides. The DNA polymerase is positioned in such a way that when the ternary complex is formed with the tagged nucleotide, the tag is captured by the nanopore and identified by the resulting current blockade signature. We first describe the construction and purification of an α-hemolysin (αHL) heptamer covalently attached to a single ϕ29 DNA polymerase using the SpyTag/SpyCatcher conjugation approach (21), followed by binding of this conjugate with template DNA and its insertion into a lipid bilayer array. We confirm that this complex is stable and retains adequate pore and polymerase activities. We verify that the tagged nucleotides developed by Fuller et al. (20) can be bound by the polymerase and accurately discriminated by the nanopore. We develop an experimental approach and computational methods to uniquely and specifically distinguish true tagged-nucleotide captures from background and from other tagged nucleotides. We address ways that tagged-nucleotide captures may be misidentified and demonstrate approaches to correct for these. We further show this protein construct can capture tagged nucleotides during template-directed DNA synthesis in the presence of Mn²⁺, demonstrating its utility for Nanopore-SBS.     

Processive DNA synthesis observed in a polymerase crystal suggests a mechanism for the prevention of frameshift mutations

DNA polymerases replicate DNA by adding nucleotides to a growing primer strand while avoiding frameshift and point mutations. Here we present a series of up to six successive replication events that were obtained by extension of a primed template directly in a crystal of the thermostable Bacillus DNA polymerase I. The 6-bp extension involves a 20-A translocation of the DNA duplex, representing the largest molecular movement observed in a protein crystal. In addition, we obtained the structure of a "closed" conformation of the enzyme with a bound triphosphate juxtaposed to a template and a dideoxy-terminated primer by constructing a point mutant that destroys a crystal lattice contact stabilizing the wild-type polymerase in an "open" conformation. Together, these observations allow many of the steps involved in DNA replication to be observed in the same enzyme at near atomic detail. The successive replication events observed directly by catalysis in the crystal confirm the general reaction sequence deduced from observations obtained by using several other polymerases and further refine critical aspects of the known reaction mechanism, and also allow us to propose new features that concern the regulated transfer of the template strand between a preinsertion site and an insertion site. We propose that such regulated transfer is an important element in the prevention of frameshift mutations in high-fidelity DNA polymerases. The ability to observe processive, high-fidelity replication directly in a crystal establishes this polymerase as a powerful model system for mechanistic studies in which the structural consequences of mismatches and DNA adducts are observed.     

Poly(ADP-Ribose): Release of Template Restriction in HeLa Cells

Evidence is presented to show that ADP-ribosylation of nuclear proteins by poly(ADP-ribose) polymerase enhances template-primer activity of HeLa cell nuclear DNA. We used Escherichia coli DNA polymerase I (EC 2.7.7.7; DNA nucleotidyltransferase) as an exogenous probe of nuclear DNA status. Neither NAD nor free poly(ADP-ribose) acts directly on the bacterial enzyme. The stimulation is specific for formation of ADP-ribosylated proteins and is not a generalized polyanion or nucleotide effect on chromatin. The release of template restriction is proportional to the capacity of a given cell line to synthesize poly(ADP-ribose); both the stimulation and poly(ADP-ribose) formation are coordinately proportional to NAD concentration. Binding studies with DNA polymerase indicate exposure or generation of additional 3'-OH primer sites due to ADP-ribosylation in intact nuclei. With intact cells, there appears a correlation among growth, physiological template restriction, and the above effects of ADP-ribosylation.