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Paper No. : 04 Genetic Engineering and Recombinant DNA Technology

Module : 14 Evolution In Enzymology: DNA Polymerases: Klenow, Taq and T7

Principal Investigator: Dr Vibha Dhawan, Distinguished Fellow and Sr. Director The Energy and Resouurces Institute (TERI), New Delhi

Co-Principal Investigator: Prof S K Jain, Professor, of Medical Biochemistry Jamia Hamdard University, New Delhi

Paper Coordinator: Dr Mohan Chandra Joshi, Assistant Professor, Jamia Millia Islamia, New Delhi

Content Writer: Dr. Bhaswati Banerjee, Assistant Professor, Gautam Buddha University, Greater Noida, UP

Content Reviwer: Dr Mohan Chandra Joshi, Assistant Professor, Jamia Millia Islamia, New Delhi

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Description of Module Subject Name Biotechnology

Paper Name Genetic Engineering and Recombinant DNA Technology

Module Name/Title

Evolution In Enzymology: DNA Polymerases: Klenow, Taq and T7 Module Id 14

Pre-requisites DNA Polymerase structure and functions

Objectives Gain comprehensive information about gradual advancement of DNA polymerases in RDT

Keywords DNA pol, Klenow, Taq, KlenTaq, T7, Sequenase

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INDEX

_____________________________________________________________________________________________________________________________

LEARNING OBJECTIVES ABOUT THIS MODULE I. INTRODUCTION

A. Enzymes used in RDT

B. DNA Polymerases: A Timeline Study

II. KLENOWPOLYMERASE-THE TRUNCATED DNA POL I A. Background

B. Structural Properties and Mode of action C. Functional Properties

D. Applications

III. TAQ POLYMERASE-THE THERMOSTABLE DNA POLYMERASE A. Background

B. Enzyme Properties and Mode of action C. KlenTaq

D. Applications

IV. T7 DNA POLYMERASE-THE SEQUENASE A. Background

B. Enzyme Properties and Mode of action C. Applications

V. SUMMARY VI. REFERENCES

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LEARNING OBJECTIVES

Enzymes used in Molecular Biology

Klenow Polymerase: Functions and Applications

Taq Polymerase: Functions and Applications

T7 Polymerase: Functions and Applications

ABOUT THIS MODULE

The focus of discussion of the present module is “evolution in enzymology”

pertaining to enzymes being used in Recombinant DNA Technology and Genetic Engineering. This module begins with comprehensive overview routinely used enzymes as molecular reagents and attempts to give an outline of enzymes used to generate and manipulate nucleic acids in vitro. Thereafter we focus on three important types of DNA polymerases routinely used in RDT: the Klenow polymerase, the Taq polymerase and the T7 polymerase. The structure, function, mode of action and applications of these enzymes are discussed in detail. A separate panel focusses on the principle of Polymerase Chain reaction. Important general applications are highlighted to wrap up the module.

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I. INTRODUCTION

We shall begin this module by addressing a very obvious question: “What is Recombinant DNA Technology”?

In the simplest terms, Recombinant DNA technology is the combined application of all of the techniques involved in the construction, study and use of recombinant DNA molecules (Khan 2010; Brown 1989). Recombinant DNA molecule is created in vitro by ligating together two different pieces of DNA that are normally not contiguous and usually obtained from two different naturally occurring (sometimes synthetic) sources. Although recombination of DNA was discovered as a naturally occurring genetic phenomenon, the first artificial or engineered recombinant DNA molecule was created in 1973 by Stanley Cohen, H. Boyer and co-workers (Stetten & DeWitt Stetten 1979; Cohen 1985; Brown 2015)

Basics of RDT: As the name suggests, the cornerstone of recombinant DNA technology is preparation of Recombinant DNA molecules and all the techniques involved therein. Recombinant DNA is mostly prepared by molecular cloning technique in order to characterize certain genes or proteins. A generalized protocol for preparation of recombinant DNA involves following fundamental steps, each of which can be manipulated or modified to small or large extent as per requirements of the experiment (Brown 2011; Murphy et al. 1984; Nicholl 2008; Primrose & Twyman 2013).

1) The primary step includes identification of the target gene or DNA sequence to be characterized i.e. “gene of interest”, considering the requirements of the experiment and feasibility of cloning, which then is known as the “insert”. The insert could be sourced from viral, prokaryotic or eukaryotic origin.

2) Selection of an appropriate DNA vehicle as carrier molecule, or the

“vector” (also known as “cloning vector”) and identifying compatible restriction sites or cloning site where the insert could be cloned into the vector comes next. The vector could be viral or prokaryotic or eukaryotic in origin

3) The insert is then introduced into the vector to prepare the vector-insert rec-DNA construct or the “clone” by a three step method: cleaving out the insert from the donor genome by restriction digestion; cutting the vector with the same set of restriction enzymes, ligating the insert into the clone by DNA ligase.

4) The rDNA molecule thus synthesized is then introduced into the host organism by a method known as “transformation” and as the host cell multiplies, it also amplifies the rDNA by producing its multiple copies autonomously thus amplifying the gene of interest.

5) Positive transformants are then selected and allowed to amplify in vitro to produce the bulk copies of the gene of interest which is then isolated by

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standard protocols to be used for further research and experimental purpose.

This protocol, although gives desired results, yet, it is quite time consuming and the time involved in this protocol may be greatly reduced by used by using in vitro DNA amplification technique such as PCR which utilizes the DNA polymerase enzymes.

It is quite obvious that a wealth of DNA modifying enzymes is routinely used in RDT to create a desired engineered rDNA. The designing and synthesis of rDNA includes various types of manipulations and modifications of DNA catalyzed by several classes of enzymes such as polymerases, nucleases, ligases, kinases, phosphatases and methylases (Primrose & Twyman 2013; Fairbanks 2004; Kendrew 2009).

A. Enzymes used in RDT

Before discussing the different DNA polymerases used in RDT, we shall have a glimpse of the important enzymes-the molecular tools- used in RDT (Fairbanks 2004; Worthington 1988; Eun 1996). As we talk about cleaving, rejoining and amplification of DNA to create an engineered DNA molecule, the enzyme that we think of first and foremost is obviously the Restriction endonuclease.

1) Restriction Endonuclease: This happens to be one of the most widely used molecular tool in RDT. Since their discovery in early 1950s by Arber, Nathans and Smith many types of restriction endonuclease enzymes have been discovered. An endonuclease is an enzyme that cleaves nucleotides within the nucleic acid molecule. Restriction endonuclease is the enzyme that recognizes only specific nucleotide sequences within the DNA (recognition site) and cuts the DNA only at specific nucleotide sequences (restriction/cleavage site) within the DNA, by breaking the phosphodiester bonds between the nucleotides.

2) DNA polymerase: An enzyme that synthesizes DNA by polymerising deoxynucleotides on a DNA template (sometimes on a RNA template in special cases) in RDT we mostly use the DNA pol-I (or a fragment thereof) which is a DNA-dependent DNA polymerase.

3) DNA ligase: The enzyme which joins compatible ends of DNA fragments by synthesizing a phosphodiester bond between adjacent nucleotides using energy from ATP hydrolysis

4) RNAse A: Nuclease that digests RNA instead of DNA

5) Exonuclease/Exonuclease III: The enzyme that cleaves DNA from the free ends of the molecule by progressive digestion of nucleotides in 3’ to 5’

direction.

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6) DNA Mtase: This enzyme adds a methyl group to a specific nucleotide in DNA.

7) Terminal Transferase: This enzyme catalyses the addition of one or more deoxyribonucleotides to the 3' terminus of the DNA molecule

8) Alkaline Phosphatase: An enzyme that removes phosphate group from the 5’ end of a DNA fragment, thus preventing non-specific and unwanted re- ligation of DNA fragments

9) Polynucleotide Kinase: This enzyme adds a phosphate group to the free 5’

terminus of a double-stranded or single-stranded DNA or RNA in an ATP dependent reaction.

10) RNA polymerase: This enzyme synthesizes new RNA in a DNA templated manner.

RDT

Restric- Endonu-tion

cleases DNA

Polymer- ases

LigaseDNA

RNAse A

Exonuc- lease III MtasesDNA

Terminal Transfe-

rases Alkaline

Phosph- atases Polynuc-

leotide kinases

RNA pol

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DNA Polymerase: The key enzymes capable of catalyzing synthesis of long chain polymers of deoxyribonucleotides in 5’ to 3’ direction in a DNA templated manner are known as DNA polymerases. Since the discovery of the DNA pol I in E. coli by Arthur Kornberg (1956), several other types of DNA polymerases have been discovered, identified and characterized in prokaryotes as well as eukaryotes (Kornberg et al.

1974; Anon 2010; Hübscher 1983). Irrespective of the origin or type of the DNA polymerase, two characteristic features are known to be consistent and conserved in all DNA pols known so far

Lack of de novo synthesis ability, hence obligatory requirement of a primer to begin DNA synthesis

5’3’ Polymerization: Synthesis or polymerization of DNA bases only in the 5’ to 3 ’direction

Most of the DNA polymerases used in RDT are obtained from the bacteria or the bacteriophage. In this small section we focus briefly on the important types of DNA Polymerases that are routinely used in molecular cloning and RDT (Brown 2011;

Brown 2015; Stetten & DeWitt Stetten 1979; Eun 1996).

Frequently Used DNA Polymerases:

DNA Poly- merase

DNA pol I

Klenow Fragment

Thermostable DNA polyme-

rases Phage DNA

polymerases Reverse

Transcri- ptase

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1) DNA Pol I: E. coli DNA polymerase I (Pol I) is the enzyme that has DNA- dependent DNA polymerase activity as well as DNA nuclease activity. It is employed for nick sealing, 5'-3' Polymerase, 3'-5' Exonuclease and 5'-3' Exonuclease activity.

2) Klenow Polymerase (large fragment of the E. coli Pol I): Klenow is the large fragment obtained by mild proteolysis of E. coli DNA polymerase I; it exhibits DNA polymerase and DNA proofreading action but lacks 5'-3' Exonuclease activity; it is used mostly in chain termination DNA sequencing reactions.

3) Taq DNA Polymerase: The thermostable DNA polymerase obtained from, the thermophilic bacteria Thermus aquaticus; mainly used for DNA synthesis by PCR.

4) Pfu Polymerase: Pfu DNA polymerase is a superior thermostable proofreading DNA polymerase enzyme isolated from the hyperthermophilic archaeon Pyrococcus furiosus; which is a high fidelity DNA polymerase used for in vitro DNA amplification by PCR.

5) Reverse Transcriptase: RNA dependent DNA polymerase used to synthesize cDNA from the mRNA template.

6) T7 DNA Polymerase: ssDNA templated DNA polymerase with unique properties; its recombinant form is commercially available as “Sequenase”.

B. DNA Polymerases: A Timeline Study

This is a comprehensive overview of the series of events from where we began to where we have reached so that we understand where we are heading to (Fig 1.1).

The enormous field of molecular biology, which forms the foundation of GE and RDT was unlocked by the pathbreaking discovery of DNA pol I in E. coli by Arthur Kornberg et. al. in 1956. Later, in 1959, Kornberg was awarded the Nobel Prize for his discovery of E. coli DNA polymerase I. The amino acid composition of Pol I was elucidated by Jovin et al. in 1969 (Worthington 1988; Stetten & DeWitt Stetten 1979). This discovery eventually changed the face of modern biology and biotechnology.

In this module we discuss a few important types of DNA polymerase that are routinely used for various RDT techniques

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II. KLENOW POLYMERASE-THE TRUNCATED DNA POL I A. Background

Escherichia coli DNA polymerase I (Pol I), one of three DNA polymerases found in E. coli, coded by the PolA gene, is the prototype DNA-dependent DNA polymerase.

DNA Pol I was the first bacterial DNA polymerase to be discovered and characterized by Arthur Kornberg in 1956. ž Pol I present in E. coli cells at a concentration of 400 molecules per cell is also the most abundantly expressed DNA polymerase in E. coli. Pol I functions in a self-regulated autonomous manner. The inherent functional features of Pol I include the following:

5′3′ polymerase activity

3′5′ exonuclease activity (for proofreading)

5′3′ exonuclease activity (for nick translation, excision repair, and hydrolysis of the RNA primers during DNA replication)

Its primary functions include filling gaps between adjacent Okazaki fragments, primer removal and proofreading during repair and replication. Pol I is known to actively participate during, DNA replication, repair and recombination and is also required for rapid, cell proliferation in rich media (Anon n.d.; Anon 2015a).

Fig. 1.1: Timeline of discovery of DNA Polymerases

Source: https://www.dna-worldwide.com/resource/160/history-dna-timeline

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Due to its characteristic inherent features, Pol I is put to several important use in RDT , which include:

 Removing 3′ protruding DNA ends (in the absence of dNTPs)

 Filling in the cohesive ends (in the presence of dNTPs) before addition of molecular linkers

But the 5’3’ exonuclease activity (involved in removal of primers) often interferes with the protocols that require only the 5’3’ polymerase activity such as copying the single-stranded DNA in dideoxy method for sequencing.

As a part of detailed structural and functional characterization of DNA pol I, Klenow and Henningsen (1970) discovered that upon mild proteolitic digestion by the protease

Subtilisin, E. coli DNA Pol I (103 kDa) produced two fragments (Fig. 2.2A, 2.2B) (Klenow & Henningsen 1970):

 a large (68 kDa) fragment known as the Klenow fragment that contains both the 5’3’ DNA polymerase and 3′5′ exonuclease (proofreading) activities

 a smaller fragment (35 kDa) that contains the 5′3′ exonuclease activity The Klenow fragment was

shown to have the capacity to synthesize new DNA strand directed by the template DNA strand but it lacked the primer removal and nick translation ability.

Thus Klenow became the enzyme of choice for the dideoxy sequencing method invented by Sanger et al (1977) and later on for several other RDT tools and techniques.

Fig. 2.1: DNA pol I-Klenow fragment Source: Biochemistry| 6th edition

Fig. 2.3A: Ribbon diagram showing crystal structure of Klenow fragment Source: http://www.pnas.org/content/95/7/3402/F1.expansion.html

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B. Structural Properties and Mode of Action

Klenow fragment was the first polymerase fragment whose 3-D structure was illustrated by X-ray crystallography. Structural and functional characterization of Klenow fragment paved the way for elucidation of structural, functional and biochemical features of DNA pol I and its mode of action as a polymerase.

Structure: Crystallographic analysis of the domain structure of Klenow fragment revealed that it is the large fragment (604 AA, 68kDa) of E. coli Pol I which corresponds to the C-terminal and the central domain of the native enzyme.

Functionally, the Klenow fragment exhibits 5’3’ polymerase and 3’5’

exonuclease activity (Klenow & Overgaard-Hansen 1970; Freemont et al. 1986) Crystal structure analysis of the Klenow fragment further reveals that in its three dimensional functionally active form Klenow resembles the shape of a human right hand where the regions resembling the fingers, palm and the thumb can be clearly identified (Fig. 2.3A, 2.3B) (Patel & Loeb 2002; Brautigam & Steitz 1998).

Mode of action: During the process of DNA polymerization, the Klenow-DNA complex is known to shuttle between the “editing mode” and the “polymerizing mode” (Brown & LiCata 2011). As the Klenow polymerase slides over the nascent DNA strand, nearly 5-8 base pairs of DNA get shifted into a large positively charged cleft for editing during which, the primer is first detached from the DNA template and then shifted to the exonuclease domain for 3’5’ exonucleolytic degradation (Fig.

2.4) (Beese et al. 1993; Beese et al. 1994).

Fig 2.3B: Structure of Klenow fragment-DNA complex Source: http://slideplayer.com/slide/5098545/

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The Klenow-DNA complex

then undergoes

intramolecular

conformational change to

“polymerizing mode” thus allowing continuation of DNA synthesis. The palm- shaped subdomain of Klenow is thought to host the catalytically active polymerization site that mediates phosphoryl transfer (Mekbib Astatke et al. 1998). Klenow polymerase fragment catalyzes the DNA

template directed addition of mononucleotides from deoxynucleoside-5′- triphosphates (dNTPs) to the free 3′-OH terminus of a primer/template DNA. Due to its inherent mechanism of action, Klenow is employed in synthesis of DNA complementary to single-stranded DNA templates (Beese et al. 1993; Wang &

Beese 2011).

Further studies have also established that a single side chain in the Klenow fragment prevents mis-incorporation of rNTPs in the nascent DNA strand (M. Astatke et al.

1998).

The “small fragment” released by mild proteolytic digestion of E. coli Pol I is a 323 aa, 35 kDa fragment that corresponds to the N-terminal domain of the native enzyme and exhibits 5’3’ Exonuclease activity. It is involved in primer removal and nick translation during DNA polymerization by the native enzyme and degradation of misincorporated DNA and RNA to monomers during DNA repair.

C. Functional Properties

Processivity: Klenow fragment shows moderate processivity. The average rate of incorporation of mono-nucleotides by Klenow fragment is 50 nucleotides per second (Beaussire & Pochet 1999; Spratt 1997; Benkovic & Cameron 1995)

Reverse Transcriptase activity: Klenow fragment has a unique functional feature.

It has been shown that Klenow polymerase may also acts as a reverse transcriptase by copying RNA templates into DNA in a distributive manner. During this RNA dependent DNA polymerization, it incorporates one to two nucleotides per binding event. However, the biological relevance of this activity is yet to be established (Ben- Mahrez et al. 1991; Bao & Cohen 2004).

Fidelity: Klenow polymerase is a DNA dependent DNA polymerase by function, which catalyzes the polymerization with fidelity approximately 1/50,000. In other

Fig: 2.4: Proofreading mechanism of Klenow fragment Source: https://www.slideshare.net/ar_shad/dna-replication-and-pcr

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words, in the event of DNA polymerization by Klenow, there occurs one mis- incorporation per 50,000 nucleotides. It is believed that 35′ editing activity of Klenow contributes to its fidelity (Beaussire & Pochet 1999; Spratt 1997; Benkovic &

Cameron 1995; van Dierendonck n.d.; Kuchta et al. 1988).

D. Applications

Due to its inherent mechanism of 5’3’ polymerase action and lack of 5’3’

exonuclease activity, Klenow is widely employed in synthesis of DNA complementary to single-stranded DNA templates thus synthesizing double stranded DNA from single stranded template.

This forms the basis of Klenow being an enzyme of choice in DNA sequencing and is predominantly used in sequencing of DNA according to Sanger et al. by the dideoxy- chain terminator method.

Other important applications in recombinant DNA technology where Klenow fragment is used are as follows:

 Synthesizing blunt DNA ends after restriction digestion: It accomplishes blunt ending of restricted DNA fragments by Removal of 3’ overhangs and fill-in of 5’ overhangs created by restriction enzymes

 For partial or complete filling up of 3´recessed ends (after restriction enzyme digestion)

 Digestion of protruding 3’ overhangs to produce blunt end

 Incorporating radiolabeled nucleotides onto 3′ DNA ends

 For random primed labeling

 Synthesizing the second strand on cDNA

 Fill-in reactions of gaps in double-stranded DNA (Nick translation)

 Elongation of oligonucleotides during the site-directed mutagenesis technique

 Conversion of 3’-recessed ends of restricted DNA fragments to blunt ends by fill-in

 3’-end-labeling of DNA fragments with -32P deoxynucleotides

 To end label the double-stranded oligonucleotides during electrophoretic mobility-shift assay (EMSA).

III. TAQ POLYMERASE-THE THERMOSTABLE DNA POLYMERASE The progress in the discipline of molecular biology and evolution of recombinant DNA Technology is starred by a number of landmark experiments and path-breaking discoveries. One such phenomenal invention is that of the PCR or the Polymerase Chain Reaction technique. A comprehensive discourse about this PCR technique is given at the end of this section of the present module.

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A. Background

PCR is a technique used to rapidly amplify a gene or a segment of a gene using repeated cycles of in vitro DNA synthesis. When the polymerase chain reaction (PCR) was originally developed by an American scientist Kary Mulis and his coworkers in 1983, the only enzyme which was being used for in vitro DNA synthesis and sequence extension was the Klenow fragment derived from the E. coli DNA Polymerase I (Mullis et al. 2012). But, like most other known enzymes at that time, the Klenow was a thermolabile enzyme that got denatured at the elevated temperatures required for PCR . Hence there was a need to replenish the enzyme before each replication and extension cycle which made the process tedious and cumbersome (Bustin 2009).

This problem was resolved only with the knowledge and application of the thermostable DNA polymerase. The first major breakthrough in this direction had come earlier when Thomas Brock and Hudson Freeze (1969) discovered a thermophilic archaebacterium thriving at extremely elevated temperatures in the hot springs and thermal vents in the Yellowstone National Park, USA (Fig. 3.1) and named it Thermus aquaticus (T. aquaticus) (Brock 1978; Brock & Freeze n.d.). As such, it was presumed that the various enzymes present in such bacteria would be resistant to elevated temperatures (temperatures greater than 45 °C). In the subsequent years, several thermostable enzymes were purified from T. aquaticus.

Chien and his coworkers (1976) isolated and characterized one of the first thermostable DNA polymerase from the T. aquaticus which was shown to retain its functional activity at elevated temperatures they named as the Taq polymerase (Chien A n.d.).

Fig. 3.1: Original habitat and source of Thermus aquaticus Source: Life at High Temperatures by Thomas D. Brock

Fig. 3.2: Crystal structure of Taq Polymerase http://www.ncbi.nlm.nih.gov/pubmed/8717047

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Eventually, Taq polymerase was one of the first thermostable DNA polymerase that was successfully used to eliminate the need to replenish enzyme after each denaturation cycle in PCR. Randall Saiki was the first scientist to employ the thermostable Taq polymerase for in vitro DNA synthesis by polymerase chain reaction. It was established that the Taq polymerase retained its functional activity at above average survival temperatures and that its halflife was nearly 40 minutes at 95C and nearly 5 minutes at 100C. This feature allowed the use of Taq polymerase as the in vitro DNA synthesizing enzyme for PCR protocols (Innis et al. 1995; Bartlett

& Stirling 2003).

B. Enzyme Properties and Mode of Action

The extensive use of Taq polymerase as the enzyme of choice for routine PCR technique may be attributed to its exceptional thermal stability and high resistance to denaturation at elevated temperatures. The structural and functional properties of Taq polymerase have been illustrated and documented through a series investigative studies including cloning, overexpression, crystallography etc. While Taq pol shows certain striking similarities with E. coli DNA pol I in terms of sequence and structure, yet, Taq has certain unique features in terms of enzyme functionality, character and dependencies which differentiate Taq pol from Pol I and make Taq the ideal polymerase for use in PCR and qPCR-based gene expression analysis. Here we summarize the important structural features of Taq polymerase and then proceed to discuss its domain structure (Urs et al. 1995; Anon 2015b; Jones 1995).

Domain Structure, Function and Fidelity:

 Cloning and characterization of Taq pol has established close resemblance of Taq pol with E. coli Pol I. These two polymerases show similar domain structure and organization in both the primary and the tertiary structural forms (Lawyer FC n.d.). Taq pol is also a single subunit DNA dependent DNA polymerase enzyme that has a full length of 832 amino acids and molecular weight of 94 kDa.

 Taq pol in its purified full length form shows a single subunit structure with two distinct domains exhibiting distinct functions. Similar to DNA pol I, the C- terminal of Taq is associated with the 5′3′ polymerase activity while the N- terminal domain of Taq exhibits 5′3′ exonuclease activity (Huang & Li 2009; Lawyer FC n.d.).

 Taq polymerase altogether lacks the 3′5′ exonuclease (proofreading) function associated with the central domain of E. coli. DNA pol I. Thus Taq pol performs DNA polymerization with a relatively moderate fidelity as it is not equipped to excise polymerization errors incurred during synthesis (Lawyer FC n.d.)

 Functionally, Taq polymerase can synthesize DNA in 5’3’ direction only and it lacks the ability of de novo DNA synthesis; thus it requires a DNA

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strand extension and achieve DNA templated DNA synthesis (Lawyer FC n.d.).

 Taq DNA polymerase exhibits a modest fidelity and is reported to polymerize dNTPs with an error rate between 1 × 10−4 and 2 × 10−5 errors per base pair, depending on experimental conditions. In routine PCR experiments, Taq is known to incur 1 error in 9000 nucleotides. It is estimated that during Taq mediated PCR, error rate of Taq pol is one error per 400 bases after 25 cycles (Lawyer FC n.d.; Eckert & Kunkel 1990; Tindall & Kunkel 1988)

Taq achieves its fidelity by the contribution of two main components:

o Discrimination against nucleotide misinsertion and o Discrimination against extension of the misinserted base

In effect, Taq shows selective preference for extending only properly matched primers and correctly paired polynucleotide sequences. The extension of incorrectly paired nucleotides by Taq occurs at a very low efficiency of 10-3 for a T-G mispair to 10-6 for A-A mispair. Further, it has also been reported that the error rate by Taq pol is approximately 10-5 for base substitution errors and 10-6 for frameshift errors (Tindall & Kunkel 1988; Elias & Andrés Cisneros 2014).

All these properties together account for the fact that Taq pol is the most obvious choice for in vitro analysis of DNA sequences and is frequently used to detect specific in vivo mutations (Eckert & Kunkel 1990).

 Another distinguishing feature of Taq DNA polymerase is its high thermostability and resistance to thermal denaturation. Biochemical analyses have revealed that Taq pol loses only 10% of its activity upon 30 minutes of incubation at 72°C and it loses 50% activity after 30 minute incubation at 95°C. Owing to the temperature dependency of Taq, its optimal functional activity is observed at 80C, unlike other enzymes whose optimal activity occurs at 37C. Taq polymerase is reported to lose half of its enzyme activity after 40 minutes of incubation at 95°C and after 5 minutes of incubation at 100°C. However, this activity span is sufficient to allow Taq pol to be used for PCR reaction, where its activity is optimized by presence of Mg2+ and a temperature of 80°C (Eckert & Kunkel 1990; Lawyer FC n.d.)

Mode of Action: Taq polymerase has been well characterized and its crystal structure has been elucidated by X-ray crystallography. Taq polymerase, in its three dimensional tertiary structure reportedly resembles the conventional “human right hand cupped structure”, a property that is common and typical for all the known DNA polymerases characterized so far. Crystal structure analyses reveal that the site for binding the single stranded-template is located in the subdomains corresponding to the fingers, binding site for the incoming dNTP is associated with the subdomain corresponding to the palm like region and the thumb shaped subdomain hosts the binding site for the dsDNA (Fig. 3.3).

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Further, it is also reported that the polymerase subdomains of Taq pol shows high degree of homology with that of the Klenow fragment and have 51% amino acid similarity. A significant deviation of the Taq from the Klenow fragment is in the fact that the 3’-5’ exonuclease domain of Taq is inactive and lacks four loops present in the Klenow fragment on one side of its 3′-5′ exonuclease site (Xu et al. 2014; Huang

& Li 2009; Lawyer FC n.d.) C. KlenTaq

Klentaq is the Klenow analog of Taq DNA polymerase. Taq pol is susceptible to proteolytic cleavage similar to DNA pol I. Upon mild protease digestion the native Taq DNA polymerase yields two distinct fragments. The smaller fragment corresponding to the N-terminal region contains the site for 5’3’ exonuclease activity.

The larger fragment corresponding to the C-terminal subdomain of Taq pol, that hosts the site for 5’3’ polymerase action is the Taq counterpart of the Klenow fragment derived from DNA pol I. Thus it is named as the “KlenTaq”or the Stoffel fragment (Marx et al. 2010; Marx et al. 2013; Wu 2015a).

Biochemical evidences indicate that KlenTaq is far more thermostable compared to the native Taq pol and it loses only 10% activity after 30 min incubation at 95 °C.

Further, KlenTaq polymerase is reported to replicate non-canonical base pairs by inducing a Watson-Crick geometry. Thus, KlenTaq proves to be a more efficient enzyme to achieve DNA amplification by polymerase chain reactions (PCR) and

Fig. 3.3: 3-D structure of Taq polymerase-DNA complex Source: https://hubpages.com/education/What-Is-Polymerase-Chain-

Reaction-PCR

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incubations at elevated temperatures and incorporation of abnormal bases (Wu 2015b)

D. Applications

Taq is a thermostable DNA dependent DNA polymerase which finds extensive applications in the Polymerase Chain reactions (PCR) which happens to be a frequently used technique in RDT and Genetic Engineering. Most of the research and industrial applications of Taq polymerase are based on its thermostability and certain unique properties.

1) Taq pol applications in PCR: PCR is a technique for in vitro amplification of genetic material, by rapidly creating multiple copies of a gene or segment of DNA using repeated cycles of in vitro DNA synthesis. PCR includes three fundamental steps: denaturation of the dsDNA template which requires extremely elevated temperature, annealing of the primer to the exposed ssDNA template which occurs at lower temperature and strand extension by the DNA polymerase. Prior to characterization and use of Taq in PCR, the process was tedious and inefficient as the E. coli DNA pol I which was used for the PCR technique was thermolabile and needed to be replenished at the beginning of every replicative cycle.

However, with the advent of Taq pol, the PCR became much simplified and seamless process, as a result several advanced versions and diverse applications of PCR could be conceptualized (Mullis et al. 2012; Erlich 1989).

Fig. 3.3: Polymerase-DNA interactions

Source: http://www.licatalab.biology.lsu.edu/andy/DNABindingEDIT.htm

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Denaturing - at 95C the H-bonds denature and dsDNA template is

opened up to provide single stranded

template

Annealing - from 45-68C the primers attach to

the homologous DNA sequence that they match Extension - Taq

polymerase begins adding

nucleotides around 72C and

extending the DNA sequence.

Fig. 3.4: Schematic representation of PCR steps Source: http://slideplayer.com.br/slide/10903472

Kary Mullis together with Michael Smith invented the path breaking technique of PCR in 1983 for which they later shared the 1993 Nobel Prize in Chemistry. But it was Randall Saiki who for the first time in 1986, employed the thermostable Taq DNA polymerase to amplify DNA segments, thus giving the PCR the mechanism that we know today. This invention revolutionized molecular biology and forensics. PCR allows amplification of trace quantities of DNA into significant amounts that can be used for experimentation or for forensic testing. Template used in PCR can be any form of DNA, and only a single molecule of DNA is needed to generate millions of copies.

PCR is based on two basic normal cellular activities:

Binding of complementary strands of DNA, and

Replication of DNA molecules by DNA polymerases

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2) PCR (and the advanced,modified versions) itself finds numerous applications in molecular biology. PCR is employed both in research and clinical investigations and industrial applications for wide range of purposes including, DNA amplification, DNA sequencing, forensic diagnostics, disease diagnostics, detection of pathogen (specially virus), detection of in vivo mutations through PCR etc [Panel-1](Biolabs 2014) 3) Taq pol used in DNA sequencing: The native Taq polymerase

discriminates against extending a DNA sequence with a misincorporated base or non-canonical base. Thus native Taq pol could not be used in Sanger’s dideoxy method for DNA sequencing as the native Taq pol incorporates dideoxynucleotide (ddNTP) with much less efficiency compared to dNTP. But with synthesis of mutated form of Taq, known as Taq Sequenase (Thermo Sequenqse) by a single substitution of substitution of Phe667Tyr in Taq pol active site, efficiency of incorporation of ddNTP by Taq was increased many folds. Thermo Sequenase is known to make “cycle sequencing” possible. It also enabled producing accurate sequences that could be analyzed by using either radioactive or fluorescent sequencing technologies (Brow 1990)

4) Taq Pol is used in T/A Cloning: Taq polymerase exhibits a deoxynucleotidyl transferase activity by which it adds a few non-templated nucleotides (usually Adenine residues) onto the the blunt 3′-end of PCR products. This property of Taq pol accounts for frequent use of Taq pol in routine molecular cloning techniques to achieve specific ligation of DNA fragments by T/A cloning protocols (Landgraf & Wolfes n.d.; Biolabs 2014). The PCR amplification of the “insert” with Taq results in synthesis of large amount of synthetic DNA with oligo-A overhangs at the 3’

terminals of both the DNA strands. When this PCR product is allowed to ligate with a linearized vector bearing 3’-T overhangs, a more specific ligation is mediated by the DNA ligase, resulting in a circular vector-insert DNA construct (Gál & Kálmán n.d.; Guo & Bi n.d.)

IV. T7 DNA POLYMERASE

DNA synthesis catalyzed by DNA polymerases is a fundamental phenomenon in terms of both in vivo processes and in vitro techniques. This happens to be one of the most investigated biological processes and almost every component involved in DNA replication has been well characterized. DNA polymerase is the master enzyme driving this process in all organisms bearing a DNA genome; as such, DNA polymerase has been a favorite with early researchers. T7 DNA polymerase is the third member of the Family A DNA polymerases, the other two members being E.

coli DNA pol I and Taq DNA polymerase (Coen n.d.; Carroll & Benkovic 1990) which we have already discussed. It is well accepted that bacteriophage T7 provides an excellent biological system to be used as a model for studying DNA synthesis

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under regulated experimental conditions, that has contributed substantially in elucidating individual steps. T7 DNA polymerase is considered to be a prototypical polymerase for characterization of the structure and function of DNA polymerase and deciphering its biochemical properties (Brown 2015; Anon 1989; Fuller 1992). Due to its flexibility in terms of incorporating desired manipulations and modifications, T7 DNA pol finds wide applications as a molecular reagent in DNA sequencing and other experiments in Recombinant DNA Technology and Genetic Engineering.

DNA sequencing enzyme|desired features

Sanger’s DNA sequencing method, also known as the Chain-terminator or Dideoxy sequencing method is by far the most frequently used method for DNA sequencing. The DNA polymerase to be used for chain-terminatior DNA sequencing, should essentially possess the following properties (Primrose & Twyman 2013):

Ability to use 2′,3′-dideoxynucleotides as substrates: The polymerase of choice must have the ability to incorporate dideoxy- and other modified nucleotides at an efficiency similar to that for the cognate deoxynucleotides;

High processivity polymerase

High fidelity in the absence of proofreading/exonuclease activity: The polymerase must be able to faithfully and completely synthesize a complementary copy of a single-stranded DNA template.

Ability to produce clear and uniform signals for detection

The DNA polymerase encoded by bacteriophage T7 is naturally endowed with or can be engineered to have all these characteristics (Richardson 1993; Fuller et al. n.d.) Most of the DNA polymerases characterized till date are known to possess the 5′→3′

exonuclease and/or the 3′→5′ exonuclease activities in addition to their polymerase activity. But both these properties are undesirable and detrimental to DNA sequencing by most of the prevalent methods. The polymerase originally used by Sanger’s dideoxy method was the Klenow fragment that was derived by proteolytically removing the 5’3’ exonuclease domain of E. coli Pol I. The major disadvantages were the presence of 3′→5′ exonuclease activity which would hydrolyze the single-stranded sequencing primers and its inherent property to discriminate against dideoxynucleotides (ddNTPs) for corresponding canonical nucleotides (dNTPs) in a sequence-dependent manner. The method was then rendered inefficient and quite expensive due to requirement of very high concentrations of ddNTPs.

Much of these problems were resolved with the use of the native Taq pol in place of Klenow fragment as Taq pol naturally lacked the 3′→5′ exonuclease activity or by the use of KlenTaq fragment which naturally lacked both the 5’3’ exonuclease and 3′→5′ exonuclease activities. However the problem of discrimination against the ddNTPs still existed.

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All these obstacles were ruled out with the discovery and characterization of the DNA polymerase from the bacteriophage T7. T7 DNA polymerase provided two immediate advantages:

 Absence of the complete 5’3’ exonuclease domain

 Lack of inherent ability to discriminate against the ddNTPs for dNTPs.

Upon further characterization, it was illustrated that this feature of non-discrimination was a result of difference in a single amino acid, tyrosine against phenylalanine.

Klenow and Taq pol are known to have Phenylalanine in their active site while T7 DNA pol contains Tyrosine in its active site which greatly reduces discrimination against ddNTPs. Further, T7 DNA Pol offered a wide scope for manipulations and modifications to obtain the DNA polymerase of precise and desired properties.

With a series of technical refinements Sanger’s DNA sequencing method has been greatly improvised and in the present day, DNA sequencing with great precision, high quality and throughput is obtained by this technique, especially when the improvements derived from 35S-labeled precursors and T7 DNA polymerase are exploited

A. Background

In the year 1971, two researchers from the Harvard Medical School, Pasquale Grippo and Charles C. Richardson identified a novel DNA polymerase of phage origin, in a mutant E. coli infected with T7 bacteriophage (Grippo & Richardson 1971). Preliminary biochemical analyses established the T7 DNA pol as a DNA templated DNA polymerase which required single stranded DNA, dNTPs and Mg2+ to be activated (Richardson 1983; Masker & Richardson 1976; Kolodner &

Richardson 1977). It was also documented that similar to E. coli DNA pol, the T7 DNA pol possesses strong 5’3’ polymerase and 3’5’ exonuclease activity. But a striking difference between the two was conspicuous absence of 5’-3’

exonuclease activity in the T7 pol unlike E. coli DNA pol I.

With further characterization and detailed investigation, certain unique and intriguing properties were observed. It was revealed that the although the structural gene for the T7 DNA pol was mapped to the gene 5 of the T7 genome, but the individual product expressed from the gene 5, known as the gp5 (84kDa), visibly lacked any DNA polymerase activity but exhibited only 3’-5’ ssDNA exonuclease activity (Hori et al. 1979b; Lee et al. 2009; Tamanot et al. 1980). It was logically speculated that the T7 DNA pol secreted by the T7 bacteriophage required a “host-component”

to reconstitute the T7 DNA polymerase holoenzyme. This “host factor’ was identified as a small 12 kDa E. coli redox protein called “Thioredoxin” (Fig. 4.1) (Mark &

Richardson 1976) (Knippers et al. 1973; Richardson 1983; Mark & Richardson 1976). Another striking revelation was that the Thioredoxin does not participate in the catalytic function of DNA polymerase through its redox capacity. T7 gp5 contributes the catalytic function to the holoenzyme while the thioredoxin plays a largely

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structural role. It essentially stabilizes the binding of gene 5 protein to a primer- template and thus increases the processivity of the polymerase more than 100- fold (Bedford et al. 1997; Tabor S n.d.).

Thus the chemically modified or the genetically engineered T7 DNA polymerase enzyme (Sequenase) revolutionized the process of DNA sequencing technology and heralded the

“Sequenase Era” (Fuller 1992; Venkitaraman 1989).

B. Enzyme Properties and Mode of Action

Enzyme Complex: T7 DNA polymerase also requires certain other accessory proteins for its DNA polymerizing action (Lee et al. 2009; Crampton & Richardson 2003). The important ones discovered so far are as follows (Fig. 4.2):

Host thioredoxin

Gp4: acts as the helicase and primase for T7 DNA polymerase

Gp2.5: functions as the single strand binding protein

Gp1.7: Nucleoside monophosphate kinase

Gp6:

Structural and Functional Properties:

Fig. 4.1: Crystal structure of T7 DNA replication complex Source:https://en.wikipedia.org/wiki/T7_DNA_polymerase

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 T7 DNA pol is equipped with extraordinary processivity for DNA polymerization as a holoenzyme i.e., after the T7 gp5 associates with the E.

coli Thioredoxin (Bedford et al. 1997)

 T7 DNA pol exhibits strong 3’-5’ Exonuclease activity by which the incorrectly paired bases are removed from the newly synthesized DNA strand. This accounts for the fidelity of DNA synthesis by T7 DNA polymerase (Grippo &

Richardson 1971)

 A notable functional property of this phage polymerase is the fact that it exhibits exonuclease activity on both single stranded and double stranded DNA. It is also known that presence of thioredoxin is essential for the double- stranded DNA exonuclease activity of T7 DNA pol (Hori et al. 1979a).

 T7 DNA pol lacks the inherent 5′–3′ exonuclease activity found in E. coli DNA polymerase I (Grippo & Richardson 1971)

Mode of Action: Putting it all together, it was confirmed that T7 DNA polymerase is an extraordinary polymerase with certain distinct features desirable for distinct applications. On account of its unique structural and functional properties, T7 DNA pol offers promising options for incorporating targeted biochemical modifications.

The high processivity and high fidelity of T7 DNA pol owing to its 3’5’

exonuclease activity are desirable features for PCR. However, 3’5’ exonuclease activity is detrimental to DNA sequencing protocols, especially the “chain termination method” of DNA sequencing. On the other hand, the inherent absence of discrimination against ddNTPs by T7 and its ability to incorporate dNTP and ddNTP with similar efficiency make it the most obvious choice as the polymerase for DNA sequencing by Sanger’s dideoxy method (Fuller et al. n.d.; Dürre &

Geriscker n.d.; Fuller 1995).

Fig. 4.2: Phage T7-DNA replication machinery Source: https://en.wikipedia.org/wiki/T7_DNA_polymerase

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With further biochemical and biophysical investigations, it was revealed that the exonuclease activity of T7 DNA polymerase could be specifically inactivated by an oxidation reaction in presence of ferrous ion and the biochemically modified T7 DNA pol (Sequenase) retained other desirable features of T7 DNA pol (Fuller 1992; Tabor

& Richardson 1987). This ruled out the only hindrance against using T7 DNA pol as the DNA sequencing polymerase, which otherwise is naturally equipped with high processivity and lack of discrimination against incorporating ddNTPs.

Thus the chemically modified or genetically engineered recombinant T7 DNA polymerase complexed with Thioredoxin, commercially available by the name

“Sequenase” with extraordinary processivity, revolutionized the progress of DNA sequencing technology and marked the onset of Sequenase Era (Fuller 1992; Fuller et al. n.d.)

C. Applications

The easily modifiable exonuclease and extraordinary processivity of T7 DNA polymerase piloted the Sequenase Era; a powerful tool in the DNA sequencing Important applications of T7 DNA polymerase are as follows:

1) Strand extensions in site directed mutagenesis

2) T7 DNA polymerase provides a biological model to study interactions within a replisome

3) The modified T7 DNA pol, the Sequenase is widely used for DNA sequencing by Sanger’s dideoxy method

4) As T7 DNA pol is a ssDNA templated DNA polymerase, it is employed for second strand synthesis of cDNA

5) It is possible to automate DNA sequencing by replacing radioactive labels with fluorescent labels

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V. SUMMARY

DNA replication – the first step in “Central Dogma” is the single most essential process for growth and proliferation of any organism

DNA polymerases catalyze the biosynthesis of DNA, a vital process for all living organisms and a process central to multiple in vitro techniques used in GE and RDT

Family A DNA polymerases include E. coli DNA polymerase I, Taq DNA polymerase and T7 DNA polymerase

Identification and isolation of these enzymes have served as prototypes for biochemical and structural studies on DNA

polymerases which are being widely used as molecular reagents

These have a pivotal role in the very foundation of RDT and Genetic

engineering and have opened up many unexplored dimensions, such as

gene cloning, DNA sequencing, PCR technology, disease detection and

diagnostics and forensic diagnostics etc.

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References

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