DNA polymerase is a specific class of enzyme found in all living organisms. Its main purpose is to replicate DNA and to help in the repair and maintenance of DNA. The enzyme is critical to the transmission of genetic information from generation to generation. Show Illustration of DNA polymerase introducing a new nucleotide into newly synthesised strand of DNA. Credit: Stephen Nakazawar Hewitt, dnaillustrated.com
Connections Arthur Kornberg | PCR ImportanceFrom the time of their discovery DNA polymerases have paved the way to new understandings of how DNA is replicated and how it is transcribed. They have also been crucial to the development of DNA sequencing and PCR, upon which much of modern biotechnology is built. Today polymerases are the core tools for DNA labelling, sequencing and amplification. DNA polymerases are also intrinsic components for the development of molecular diagnostics for personalised medicine. They are at the forefront, for example, of techniques to detect genomic alterations that can cause diseases like cancer or cause patients to experience adverse reactions to drugs. DiscoveryThe history of DNA polymerase is rooted in the work of Arthur Kornberg who in 1948 discovered that an enzyme he extracted from potatoes (nucleotide pyrophosphatase) could synthesise Nicotinamide adenine dinucleotide (NAD), a coenzyme found in all living cells. Soon after he discovered more enzymes that could synthesise other coenzymes. Having found an enzyme that incorporated a nucleotide into a coenzyme, in 1950 Kornberg began to investigate whether there were other enzymes that could assemble the many nucleotides that make up the chains of nucleic acids, particularly RNA. The difficulty he faced was that at this stage scientists did not know what the actual building blocks of nucleic acids were.By 1954 Kornberg's research and that of others had shown that the likely nucleotide building blocks of nucleic acids were synthesised and activated in cells. Based on this he and his post-doctoral fellow Uri Littauer, launched some experiments to test the power of extracts from Escherichia coli (E coli) to synthesise RNA. This they performed with a radiolabelled coenzyme called adenosine triphosphate (ATP). Their work, however, was to take a new direction when, in 1955, Mariane Grunberg-Manago, a postdoctoral fellow in Severo Ochaoa's laboratory, announced the identification of a new enzyme, polynucleotide phosphorylase. This she had stumbled upon while conducting research to understand aerobic phosphorylation in extracts of Azotobacter vinelandii, a soil dwelling organism. Importantly, the enzyme was shown to be capable of synthesising RNA in a test tube from simple nucleotides. This she had done with the use of the coenzyme Adenosine diphosphate (ADP). Thereafter, Kornberg shifted his efforts to using ADP in his experiments and hunting for an enzyme that could build DNA. By 1956 he and his colleagues, including Robert Lehman, had purified DNA polymerase from E coli extracts. This had been a difficult and demanding task, hindered by the fact that only relatively small amounts of the enzyme could be extracted from E coli. Their work was helped by the recent installation of a fermentor in the department for the growth of E coli which supplied hundreds of grams of log phase E coli. With the aid of chromatography Kornberg's team was able to obtain a several thousand-fold purified but not yet homogeneous preparation of DNA polymerase. While still impure, the enzyme proved capable of DNA replication. This DNA synthesis was aided by the addition of DNA as a primer. Today the enzyme that Kornberg and colleagues purified is called DNA polymerase I. Many other DNA polymerases have been isolated from E coli since the 1950s, two of them identified by Kornberg's son, Thomas. DNA polymerases have also been purified from other bacteria. This includes Taq DNA polymerase purified from the bacterium Thermus aquaticus in 1976 which was found to live in the hot springs of Yellowstone Park in Wyoming by Thomas Brock in 1965. The advantage of Taq is that it can withstand very high temperatures. This makes it suitable for use in PCR. ApplicationPolymerase does not create a novel DNA strand from scratch. Instead it synthesizes a new strand of DNA based on the template of two existing DNA strands. It does this with the help of another enzyme, called helicase, which unwinds the double helix structure of the DNA molecule into two single DNA strands. In addition to a template strand, polymerases require a primer to function. This is a fragment of nucleic acid that serves as the starting point for DNA replication. The primer, often a short strand of RNA, needs to be complementary to the template. DNA polymerase works by sliding along the single strand template of DNA reading its nucleotide bases as it goes along and inserting new complementary nucleotides into the primer so as to make a sequence complementary to the template. DNA polymerase is thought to be able to replicate 749 nucleotides per second. By the end of the replication process two new DNA molecules will have been made, each identical to the other and to the original parent molecule. Such accurate replication is helped by the fact that DNA polymerase has an inbuilt capacity to detect and correct any mistakes it makes in the replication process. Several families of DNA polymerases have now been identified and new ones are continuing to be discovered. Some of the most useful polymerases for biotechnology are those classified in families labelled A and B. These tend to be single subunit polymerases. Genetic engineering is also adding tailor-made polymerases to the repertoire. Such genetically tailored DNA polymerases have helped increase the speed and accuracy of PCR and enable PCR to be carried out directly from tissue (ie blood). They have also facilitated the development of whole genome amplification and the generation of next generation sequencing tools. DNA polymerase: timeline of key events
The preliminary finding was announced at the annual meeting of the Federation of American Societies for Experimental Biology. It was achieved by Arthur Kornberg, an American biochemist, and his colleagues while studying Escherichia coli, a type of bacteria. The discovery that DNA polymerase, an enzyme, could replicate DNA was a major breakthrough because up to this point most scientists believed it was not possible for scientists to duplicate the genetic specificity that is required for DNA replication outside of an intact cell. Kornberg's work opened the way to the discovery of many other similar enzymes and the development of recombinant DNA. The work was published in A Kornberg, I R Lehman, E S Simms, 'Polydesoxyribonucleotide synthesis by enzymes from Escherichia coli', Fed Proc 15 (1956), 291.1956-04-16T00:00:00+0000
DNA polymerase isolated and purified and shown to replicate DNA We use cookies to enhance your experience. By continuing to browse this site you agree to our use of cookies. More info. 3D structure of the DNA-binding helix-turn-helix motifs in human DNA polymerase beta (based on PDB file 7ICG) IdentifiersEC no.2.7.7.7CAS no.9012-90-2 DatabasesIntEnzIntEnz viewBRENDABRENDA entryExPASyNiceZyme viewKEGGKEGG entryMetaCycmetabolic pathwayPRIAMprofilePDB structuresRCSB PDB PDBe PDBsumGene OntologyAmiGO / QuickGO
A DNA polymerase is a member of a family of enzymes that catalyze the synthesis of DNA molecules from nucleoside triphosphates, the molecular precursors of DNA. These enzymes are essential for DNA replication and usually work in groups to create two identical DNA duplexes from a single original DNA duplex. During this process, DNA polymerase "reads" the existing DNA strands to create two new strands that match the existing ones.[1][2][3][4][5][6] These enzymes catalyze the chemical reaction deoxynucleoside triphosphate + DNAn ⇌ pyrophosphate + DNAn+1.DNA polymerase adds nucleotides to the three prime (3')-end of a DNA strand, one nucleotide at a time. Every time a cell divides, DNA polymerases are required to duplicate the cell's DNA, so that a copy of the original DNA molecule can be passed to each daughter cell. In this way, genetic information is passed down from generation to generation. Before replication can take place, an enzyme called helicase unwinds the DNA molecule from its tightly woven form, in the process breaking the hydrogen bonds between the nucleotide bases. This opens up or "unzips" the double-stranded DNA to give two single strands of DNA that can be used as templates for replication in the above reaction. HistoryIn 1956, Arthur Kornberg and colleagues discovered DNA polymerase I (Pol I), in Escherichia coli. They described the DNA replication process by which DNA polymerase copies the base sequence of a template DNA strand. Kornberg was later awarded the Nobel Prize in Physiology or Medicine in 1959 for this work.[7] DNA polymerase II was discovered by Thomas Kornberg (the son of Arthur Kornberg) and Malcolm E. Gefter in 1970 while further elucidating the role of Pol I in E. coli DNA replication.[8] Three more DNA polymerases have been found in E. coli, including DNA polymerase III (discovered in the 1970s) and DNA polymerases IV and V (discovered in 1999).[9] FunctionThe main function of DNA polymerase is to synthesize DNA from deoxyribonucleotides, the building blocks of DNA. The DNA copies are created by the pairing of nucleotides to bases present on each strand of the original DNA molecule. This pairing always occurs in specific combinations, with cytosine along with guanine, and thymine along with adenine, forming two separate pairs, respectively. By contrast, RNA polymerases synthesize RNA from ribonucleotides from either RNA or DNA. When synthesizing new DNA, DNA polymerase can add free nucleotides only to the 3' end of the newly forming strand. This results in elongation of the newly forming strand in a 5'–3' direction. It is important to note that the directionality of the newly forming strand (the daughter strand) is opposite to the direction in which DNA polymerase moves along the template strand. Since DNA polymerase requires a free 3' OH group for initiation of synthesis, it can synthesize in only one direction by extending the 3' end of the preexisting nucleotide chain. Hence, DNA polymerase moves along the template strand in a 3'–5' direction, and the daughter strand is formed in a 5'–3' direction. This difference enables the resultant double-strand DNA formed to be composed of two DNA strands that are antiparallel to each other. The function of DNA polymerase is not quite perfect, with the enzyme making about one mistake for every billion base pairs copied. Error correction is a property of some, but not all DNA polymerases. This process corrects mistakes in newly synthesized DNA. When an incorrect base pair is recognized, DNA polymerase moves backwards by one base pair of DNA. The 3'–5' exonuclease activity of the enzyme allows the incorrect base pair to be excised (this activity is known as proofreading). Following base excision, the polymerase can re-insert the correct base and replication can continue forwards. This preserves the integrity of the original DNA strand that is passed onto the daughter cells. Fidelity is very important in DNA replication. Mismatches in DNA base pairing can potentially result in dysfunctional proteins and could lead to cancer. Many DNA polymerases contain an exonuclease domain, which acts in detecting base pair mismatches and further performs in the removal of the incorrect nucleotide to be replaced by the correct one.[10] The shape and the interactions accommodating the Watson and Crick base pair are what primarily contribute to the detection or error. Hydrogen bonds play a key role in base pair binding and interaction. The loss of an interaction, which occurs at a mismatch, is said to trigger a shift in the balance, for the binding of the template-primer, from the polymerase, to the exonuclease domain. In addition, an incorporation of a wrong nucleotide causes a retard in DNA polymerization. This delay gives time for the DNA to be switched from the polymerase site to the exonuclease site. Different conformational changes and loss of interaction occur at different mismatches. In a purine:pyrimidine mismatch there is a displacement of the pyrimidine towards the major groove and the purine towards the minor groove. Relative to the shape of DNA polymerase's binding pocket, steric clashes occur between the purine and residues in the minor groove, and important van der Waals and electrostatic interactions are lost by the pyrimidine.[11] Pyrimidine:pyrimidine and purine:purine mismatches present less notable changes since the bases are displaced towards the major groove, and less steric hindrance is experienced. However, although the different mismatches result in different steric properties, DNA polymerase is still able to detect and differentiate them so uniformly and maintain fidelity in DNA replication.[12] DNA polymerization is also critical for many mutagenesis processes and is widely employed in biotechnologies. StructureThe known DNA polymerases have highly conserved structure, which means that their overall catalytic subunits vary very little from species to species, independent of their domain structures. Conserved structures usually indicate important, irreplaceable functions of the cell, the maintenance of which provides evolutionary advantages. The shape can be described as resembling a right hand with thumb, finger, and palm domains. The palm domain appears to function in catalyzing the transfer of phosphoryl groups in the phosphoryl transfer reaction. DNA is bound to the palm when the enzyme is active. This reaction is believed to be catalyzed by a two-metal-ion mechanism. The finger domain functions to bind the nucleoside triphosphates with the template base. The thumb domain plays a potential role in the processivity, translocation, and positioning of the DNA.[13] ProcessivityDNA polymerase's rapid catalysis is due to its processive nature. Processivity is a characteristic of enzymes that function on polymeric substrates. In the case of DNA polymerase, the degree of processivity refers to the average number of nucleotides added each time the enzyme binds a template. The average DNA polymerase requires about one second locating and binding a primer/template junction. Once it is bound, a nonprocessive DNA polymerase adds nucleotides at a rate of one nucleotide per second.[14]: 207–208 Processive DNA polymerases, however, add multiple nucleotides per second, drastically increasing the rate of DNA synthesis. The degree of processivity is directly proportional to the rate of DNA synthesis. The rate of DNA synthesis in a living cell was first determined as the rate of phage T4 DNA elongation in phage infected E. coli. During the period of exponential DNA increase at 37 °C, the rate was 749 nucleotides per second.[15] DNA polymerase's ability to slide along the DNA template allows increased processivity. There is a dramatic increase in processivity at the replication fork. This increase is facilitated by the DNA polymerase's association with proteins known as the sliding DNA clamp. The clamps are multiple protein subunits associated in the shape of a ring. Using the hydrolysis of ATP, a class of proteins known as the sliding clamp loading proteins open up the ring structure of the sliding DNA clamps allowing binding to and release from the DNA strand. Protein–protein interaction with the clamp prevents DNA polymerase from diffusing from the DNA template, thereby ensuring that the enzyme binds the same primer/template junction and continues replication.[14]: 207–208 DNA polymerase changes conformation, increasing affinity to the clamp when associated with it and decreasing affinity when it completes the replication of a stretch of DNA to allow release from the clamp. Variation across speciesc:o6-methyl-guanine pair in the polymerase-2 basepair position IdentifiersSymbolDNA_pol_APfamPF00476InterProIPR001098SMART-PROSITEPDOC00412SCOP21dpi / SCOPe / SUPFAM
crystal structure of rb69 gp43 in complex with dna containing thymine glycol IdentifiersSymbolDNA_pol_BPfamPF00136Pfam clanCL0194InterProIPR006134PROSITEPDOC00107SCOP21noy / SCOPe / SUPFAM
phi29 dna polymerase, orthorhombic crystal form, ssdna complex IdentifiersSymbolDNA_pol_B_2PfamPF03175Pfam clanCL0194InterProIPR004868
Based on sequence homology, DNA polymerases can be further subdivided into seven different families: A, B, C, D, X, Y, and RT. Some viruses also encode special DNA polymerases, such as Hepatitis B virus DNA polymerase. These may selectively replicate viral DNA through a variety of mechanisms. Retroviruses encode an unusual DNA polymerase called reverse transcriptase, which is an RNA-dependent DNA polymerase (RdDp). It polymerizes DNA from a template of RNA.
Prokaryotic polymeraseProkaryotic polymerases exist in two forms: core polymerase and holoenzyme. Core polymerase synthesizes DNA from the DNA template but it cannot initiate the synthesis alone or accurately. Holoenzyme accurately initiates synthesis. Pol IProkaryotic family A polymerases include the DNA polymerase I (Pol I) enzyme, which is encoded by the polA gene and ubiquitous among prokaryotes. This repair polymerase is involved in excision repair with both 3'–5' and 5'–3' exonuclease activity and processing of Okazaki fragments generated during lagging strand synthesis.[20] Pol I is the most abundant polymerase, accounting for >95% of polymerase activity in E. coli; yet cells lacking Pol I have been found suggesting Pol I activity can be replaced by the other four polymerases. Pol I adds ~15-20 nucleotides per second, thus showing poor processivity. Instead, Pol I starts adding nucleotides at the RNA primer:template junction known as the origin of replication (ori). Approximately 400 bp downstream from the origin, the Pol III holoenzyme is assembled and takes over replication at a highly processive speed and nature.[21] Taq polymerase is a heat-stable enzyme of this family that lacks proofreading ability.[22] Pol IIDNA polymerase II is a family B polymerase encoded by the polB gene. Pol II has 3'–5' exonuclease activity and participates in DNA repair, replication restart to bypass lesions, and its cell presence can jump from ~30-50 copies per cell to ~200–300 during SOS induction. Pol II is also thought to be a backup to Pol III as it can interact with holoenzyme proteins and assume a high level of processivity. The main role of Pol II is thought to be the ability to direct polymerase activity at the replication fork and help stalled Pol III bypass terminal mismatches.[23] Pfu DNA polymerase is a heat-stable enzyme of this family found in the hyperthermophilic archaeon Pyrococcus furiosus.[24] Detailed classification divides family B in archaea into B1, B2, B3, in which B2 is a group of pseudoenzymes. Pfu belongs to family B3. Others PolBs found in archaea are part of "Casposons", Cas1-dependent transposons.[25] Some viruses (including Φ29 DNA polymerase) and mitochondrial plasmids carry polB as well.[26] Pol IIIDNA polymerase III holoenzyme is the primary enzyme involved in DNA replication in E. coli and belongs to family C polymerases. It consists of three assemblies: the pol III core, the beta sliding clamp processivity factor, and the clamp-loading complex. The core consists of three subunits: α, the polymerase activity hub, ɛ, exonucleolytic proofreader, and θ, which may act as a stabilizer for ɛ. The beta sliding clamp processivity factor is also present in duplicate, one for each core, to create a clamp that encloses DNA allowing for high processivity.[27] The third assembly is a seven-subunit (τ2γδδ′χψ) clamp loader complex. The old textbook "trombone model" depicts an elongation complex with two equivalents of the core enzyme at each replication fork (RF), one for each strand, the lagging and leading.[23] However, recent evidence from single-molecule studies indicates an average of three stoichiometric equivalents of core enzyme at each RF for both Pol III and its counterpart in B. subtilis, PolC.[28] In-cell fluorescent microscopy has revealed that leading strand synthesis may not be completely continuous, and Pol III* (i.e., the holoenzyme α, ε, τ, δ and χ subunits without the ß2 sliding clamp) has a high frequency of dissociation from active RFs.[29] In these studies, the replication fork turnover rate was about 10s for Pol III*, 47s for the ß2 sliding clamp, and 15m for the DnaB helicase. This suggests that the DnaB helicase may remain stably associated at RFs and serve as a nucleation point for the competent holoenzyme. In vitro single-molecule studies have shown that Pol III* has a high rate of RF turnover when in excess, but remains stably associated with replication forks when concentration is limiting.[29] Another single-molecule study showed that DnaB helicase activity and strand elongation can proceed with decoupled, stochastic kinetics.[29] Pol IVIn E. coli, DNA polymerase IV (Pol IV) is an error-prone DNA polymerase involved in non-targeted mutagenesis.[30] Pol IV is a Family Y polymerase expressed by the dinB gene that is switched on via SOS induction caused by stalled polymerases at the replication fork. During SOS induction, Pol IV production is increased tenfold and one of the functions during this time is to interfere with Pol III holoenzyme processivity. This creates a checkpoint, stops replication, and allows time to repair DNA lesions via the appropriate repair pathway.[31] Another function of Pol IV is to perform translesion synthesis at the stalled replication fork like, for example, bypassing N2-deoxyguanine adducts at a faster rate than transversing undamaged DNA. Cells lacking the dinB gene have a higher rate of mutagenesis caused by DNA damaging agents.[32] Pol VDNA polymerase V (Pol V) is a Y-family DNA polymerase that is involved in SOS response and translesion synthesis DNA repair mechanisms.[33] Transcription of Pol V via the umuDC genes is highly regulated to produce only Pol V when damaged DNA is present in the cell generating an SOS response. Stalled polymerases causes RecA to bind to the ssDNA, which causes the LexA protein to autodigest. LexA then loses its ability to repress the transcription of the umuDC operon. The same RecA-ssDNA nucleoprotein posttranslationally modifies the UmuD protein into UmuD' protein. UmuD and UmuD' form a heterodimer that interacts with UmuC, which in turn activates umuC's polymerase catalytic activity on damaged DNA.[34] In E. coli, a polymerase "tool belt" model for switching pol III with pol IV at a stalled replication fork, where both polymerases bind simultaneously to the β-clamp, has been proposed.[35] However, the involvement of more than one TLS polymerase working in succession to bypass a lesion has not yet been shown in E. coli. Moreover, Pol IV can catalyze both insertion and extension with high efficiency, whereas pol V is considered the major SOS TLS polymerase. One example is the bypass of intra strand guanine thymine cross-link where it was shown on the basis of the difference in the mutational signatures of the two polymerases, that pol IV and pol V compete for TLS of the intra-strand crosslink.[35] Family DIn 1998, the family D of DNA polymerase was discovered in Pyrococcus furiosus and Methanococcus jannaschii.[37] The PolD complex is a heterodimer of two chains, each encoded by DP1 (small proofreading) and DP2 (large catalytic). Unlike other DNA polymerases, the structure and mechanism of the DP2 catalytic core resemble that of multi-subunit RNA polymerases. The DP1-DP2 interface resembles that of Eukaryotic Class B polymerase zinc finger and its small subunit.[17] DP1, a Mre11-like exonuclease,[38] is likely the precursor of small subunit of Pol α and ε, providing proofreading capabilities now lost in Eukaryotes.[25] Its N-terminal HSH domain is similar to AAA proteins, especially Pol III subunit δ and RuvB, in structure.[39] DP2 has a Class II KH domain.[17] Pyrococcus abyssi polD is more heat-stable and more accurate than Taq polymerase, but has not yet been commercialized.[40] It has been proposed that family D DNA polymerase was the first to evolve in cellular organisms and that the replicative polymerase of the Last Universal Cellular Ancestor (LUCA) belonged to family D.[41] Eukaryotic DNA polymerasePolymerases β, λ, σ, μ (beta, lambda, sigma, mu) and TdTFamily X polymerases contain the well-known eukaryotic polymerase pol β (beta), as well as other eukaryotic polymerases such as Pol σ (sigma), Pol λ (lambda), Pol μ (mu), and Terminal deoxynucleotidyl transferase (TdT). Family X polymerases are found mainly in vertebrates, and a few are found in plants and fungi. These polymerases have highly conserved regions that include two helix-hairpin-helix motifs that are imperative in the DNA-polymerase interactions. One motif is located in the 8 kDa domain that interacts with downstream DNA and one motif is located in the thumb domain that interacts with the primer strand. Pol β, encoded by POLB gene, is required for short-patch base excision repair, a DNA repair pathway that is essential for repairing alkylated or oxidized bases as well as abasic sites. Pol λ and Pol μ, encoded by the POLL and POLM genes respectively, are involved in non-homologous end-joining, a mechanism for rejoining DNA double-strand breaks due to hydrogen peroxide and ionizing radiation, respectively. TdT is expressed only in lymphoid tissue, and adds "n nucleotides" to double-strand breaks formed during V(D)J recombination to promote immunological diversity.[42] Polymerases α, δ and ε (alpha, delta, and epsilon)Pol α (alpha), Pol δ (delta), and Pol ε (epsilon) are members of Family B Polymerases and are the main polymerases involved with nuclear DNA replication. Pol α complex (pol α-DNA primase complex) consists of four subunits: the catalytic subunit POLA1, the regulatory subunit POLA2, and the small and the large primase subunits PRIM1 and PRIM2 respectively. Once primase has created the RNA primer, Pol α starts replication elongating the primer with ~20 nucleotides.[43] Due to its high processivity, Pol δ takes over the leading and lagging strand synthesis from Pol α.[14]: 218–219 Pol δ is expressed by genes POLD1, creating the catalytic subunit, POLD2, POLD3, and POLD4 creating the other subunits that interact with Proliferating Cell Nuclear Antigen (PCNA), which is a DNA clamp that allows Pol δ to possess processivity.[44] Pol ε is encoded by the POLE1, the catalytic subunit, POLE2, and POLE3 gene. It has been reported that the function of Pol ε is to extend the leading strand during replication,[45][46] while Pol δ primarily replicates the lagging strand; however, recent evidence suggested that Pol δ might have a role in replicating the leading strand of DNA as well.[47] Pol ε's C-terminus "polymerase relic" region, despite being unnecessary for polymerase activity,[48] is thought to be essential to cell vitality. The C-terminus region is thought to provide a checkpoint before entering anaphase, provide stability to the holoenzyme, and add proteins to the holoenzyme necessary for initiation of replication.[49] Pol ε has a larger "palm" domain that provides high processivity independently of PCNA.[48] Compared to other Family B polymerases, the DEDD exonuclease family responsible for proofreading is inactivated in Pol α.[25] Pol ε is unique in that it has two zinc finger domains and an inactive copy of another family B polymerase in its C-terminal. The presence of this zinc finger has implications in the origins of Eukaryota, which in this case is placed into the Asgard group with archaeal B3 polymerase.[50] Polymerases η, ι and κ (eta, iota, and kappa)Pol η (eta), Pol ι (iota), and Pol κ (kappa), are Family Y DNA polymerases involved in the DNA repair by translation synthesis and encoded by genes POLH, POLI, and POLK respectively. Members of Family Y have five common motifs to aid in binding the substrate and primer terminus and they all include the typical right hand thumb, palm and finger domains with added domains like little finger (LF), polymerase-associated domain (PAD), or wrist. The active site, however, differs between family members due to the different lesions being repaired. Polymerases in Family Y are low-fidelity polymerases, but have been proven to do more good than harm as mutations that affect the polymerase can cause various diseases, such as skin cancer and Xeroderma Pigmentosum Variant (XPS). The importance of these polymerases is evidenced by the fact that gene encoding DNA polymerase η is referred as XPV, because loss of this gene results in the disease Xeroderma Pigmentosum Variant. Pol η is particularly important for allowing accurate translesion synthesis of DNA damage resulting from ultraviolet radiation. The functionality of Pol κ is not completely understood, but researchers have found two probable functions. Pol κ is thought to act as an extender or an inserter of a specific base at certain DNA lesions. All three translesion synthesis polymerases, along with Rev1, are recruited to damaged lesions via stalled replicative DNA polymerases. There are two pathways of damage repair leading researchers to conclude that the chosen pathway depends on which strand contains the damage, the leading or lagging strand.[51] Polymerases Rev1 and ζ (zeta)Pol ζ another B family polymerase, is made of two subunits Rev3, the catalytic subunit, and Rev7 (MAD2L2), which increases the catalytic function of the polymerase, and is involved in translesion synthesis. Pol ζ lacks 3' to 5' exonuclease activity, is unique in that it can extend primers with terminal mismatches. Rev1 has three regions of interest in the BRCT domain, ubiquitin-binding domain, and C-terminal domain and has dCMP transferase ability, which adds deoxycytidine opposite lesions that would stall replicative polymerases Pol δ and Pol ε. These stalled polymerases activate ubiquitin complexes that in turn disassociate replication polymerases and recruit Pol ζ and Rev1. Together Pol ζ and Rev1 add deoxycytidine and Pol ζ extends past the lesion. Through a yet undetermined process, Pol ζ disassociates and replication polymerases reassociate and continue replication. Pol ζ and Rev1 are not required for replication, but loss of REV3 gene in budding yeast can cause increased sensitivity to DNA-damaging agents due to collapse of replication forks where replication polymerases have stalled.[52] TelomeraseTelomerase is a ribonucleoprotein which functions to replicate ends of linear chromosomes since normal DNA polymerase cannot replicate the ends, or telomeres. The single-strand 3' overhang of the double-strand chromosome with the sequence 5'-TTAGGG-3' recruits telomerase. Telomerase acts like other DNA polymerases by extending the 3' end, but, unlike other DNA polymerases, telomerase does not require a template. The TERT subunit, an example of a reverse transcriptase, uses the RNA subunit to form the primer–template junction that allows telomerase to extend the 3' end of chromosome ends. The gradual decrease in size of telomeres as the result of many replications over a lifetime are thought to be associated with the effects of aging.[14]: 248–249 Polymerases γ, θ and ν (gamma, theta and nu)Pol γ (gamma), Pol θ (theta), and Pol ν (nu) are Family A polymerases. Pol γ, encoded by the POLG gene, was long thought to be the only mitochondrial polymerase. However, recent research shows that at least Pol β (beta), a Family X polymerase, is also present in mitochondria.[53][54] Any mutation that leads to limited or non-functioning Pol γ has a significant effect on mtDNA and is the most common cause of autosomal inherited mitochondrial disorders.[55] Pol γ contains a C-terminus polymerase domain and an N-terminus 3'–5' exonuclease domain that are connected via the linker region, which binds the accessory subunit. The accessory subunit binds DNA and is required for processivity of Pol γ. Point mutation A467T in the linker region is responsible for more than one-third of all Pol γ-associated mitochondrial disorders.[56] While many homologs of Pol θ, encoded by the POLQ gene, are found in eukaryotes, its function is not clearly understood. The sequence of amino acids in the C-terminus is what classifies Pol θ as Family A polymerase, although the error rate for Pol θ is more closely related to Family Y polymerases. Pol θ extends mismatched primer termini and can bypass abasic sites by adding a nucleotide. It also has Deoxyribophosphodiesterase (dRPase) activity in the polymerase domain and can show ATPase activity in close proximity to ssDNA.[57] Pol ν (nu) is considered to be the least effective of the polymerase enzymes.[58] However, DNA polymerase nu plays an active role in homology repair during cellular responses to crosslinks, fulfilling its role in a complex with helicase.[58] Plants use two Family A polymerases to copy both the mitochondrial and plastid genomes. They are more similar to bacterial Pol I than they are to mammalian Pol γ.[59] Reverse transcriptaseRetroviruses encode an unusual DNA polymerase called reverse transcriptase, which is an RNA-dependent DNA polymerase (RdDp) that synthesizes DNA from a template of RNA. The reverse transcriptase family contain both DNA polymerase functionality and RNase H functionality, which degrades RNA base-paired to DNA. An example of a retrovirus is HIV.[14] Reverse transcriptase is commonly employed in amplification of RNA for research purposes. Using an RNA template, PCR can utilize reverse transcriptase, creating a DNA template. This new DNA template can then be used for typical PCR amplification. The products of such an experiment are thus amplified PCR products from RNA.[9] Each HIV retrovirus particle contains two RNA genomes, but, after an infection, each virus generates only one provirus.[60] After infection, reverse transcription is accompanied by template switching between the two genome copies (copy choice recombination).[60] From 5 to 14 recombination events per genome occur at each replication cycle.[61] Template switching (recombination) appears to be necessary for maintaining genome integrity and as a repair mechanism for salvaging damaged genomes.[62][60] Bacteriophage T4 DNA polymeraseBacteriophage (phage) T4 encodes a DNA polymerase that catalyzes DNA synthesis in a 5' to 3' direction.[63] The phage polymerase also has an exonuclease activity that acts in a 3' to 5' direction,[64] and this activity is employed in the proofreading and editing of newly inserted bases.[65] A phage mutant with a temperature sensitive DNA polymerase, when grown at permissive temperatures, was observed to undergo recombination at frequencies that are about two-fold higher than that of wild-type phage.[66] It was proposed that a mutational alteration in the phage DNA polymerase can stimulate template strand switching (copy choice recombination) during replication.[66] See also
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