ArticleslgStudy

chemistry

T7 DNA polymerase

T7 DNA polymerase is a chemistry topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand T7 DNA polymerase rather than just read about it. In short: T7 DNA polymerase is an enzyme used during the DNA replication of the T7 bacteriophage. During this process, the DNA polymerase “reads” existing DNA strands and creates two new strands that match the existing ones.

T7 DNA polymerase — main illustration
T7 DNA polymerase — illustration

Key takeaways

  • T7 DNA polymerase belongs to chemistry; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect T7 DNA polymerase to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of T7 DNA polymerase from memory before moving on to harder problems.

Reference excerpt

T7 DNA polymerase is an enzyme used during the DNA replication of the T7 bacteriophage. During this process, the DNA polymerase “reads” existing DNA strands and creates two new strands that match the existing ones. The T7 DNA polymerase requires a host factor, E. coli thioredoxin, in order to carry out its function. This helps stabilize the binding of the necessary protein to the primer-template to improve processivity by more than 100-fold, which is a feature unique to this enzyme. It is a member of the Family A DNA polymerases, which include E. coli DNA polymerase I and Taq DNA polymerase. This polymerase has various applications in site-directed mutagenesis as well as a high-fidelity enzyme suitable for PCR. It has also served as the precursor to Sequenase, an engineered-enzyme optimized for DNA sequencing.

Mechanism

Phosphoryl transfer

Figure 2. Nucleotidyl transfer by DNA polymerase. T7 DNA polymerase catalyzes the phosphoryl transfer during DNA replication of the T7 phage. As shown in Figure 2, the 3’ hydroxyl group of a primer acts as a nucleophile and attacks the phosphodiester bond of nucleoside 5’-triphosphate (dTMP-PP). This reaction adds a nucleoside monophosphate into DNA and releases a pyrophosphate (PPi). Generally, the reaction is metal-dependent and cations such as Mg2+ are often present in the enzyme active site. For T7 DNA polymerase, the fingers, palm and thumb (Figure 1) position the primer-template so that the 3’-end of the primer strand is positioned next to the nucleotide-binding site (located at the intersection of the fingers and thumb). The base pair formed between the nucleotide and the template base fits nicely into a groove between the fingers and the 3’-end of the primer. Two Mg2+ ions form an octahedral coordinate network with oxygen ligand and also bring the reactive primer hydroxyl and the nucleotide α-phosphate close together, thereby lowering the entropic cost of nucleophilic addition. The rate-limiting step in the catalytic cycle occurs after the nucleoside triphosphate binds and before it is incorporated into the DNA (corresponding to the closure of the fingers subdomain around the DNA and nucleotide).

Role of Mg2+ ions and amino acid residues in the active site The amino acids present in the active site assist in creating a stabilizing environment for the reaction to proceed. Amino acids such as Lys522, Tyr526, His506 and Arg518 act as hydrogen bond donors. The backbone carbonyl of Ala476, Asp475 and Asp654 form coordinate bonds with the Mg2+ ions. Asp475 and Asp654 form a bridge with the Mg2+ cations to orient them properly. The Mg2+ ion on the right (Figure 3) interacts with negatively charged oxygens of the alpha(α), beta(β) and gamma(γ) phosphates to align the scissile bond for the primer to attack. Even if there is no general base within the active site to deprotonate the primer hydroxyl, the lowered pka of the metal-bound hydroxyl favors the formation of the 3’-hydroxide nucleophile. Metal ions and Lys522 contact non-bridging oxygens on the α-phosphate to stabilize the negative charge developing on the α-phosphorus during bond formation with the nucleophile. Moreover, the Lys522 sidechain also moves to neutralize the negatively charged pyrophosphate group. Tyr526, His506, Arg518 side chains and the oxygen from the backbone carbonyl group of Ala476 take part in the hydrogen bond network and assist in aligning the substrate for phosphoryl transfer.

Accessory proteins

While phage T7 mediates DNA replication in very similar manner to higher organisms, T7 system is generally simpler compared to other replication systems. In addition to T7 DNA polymerase (also known as gp5), T7 replisome requires only four accessory proteins for proper function: host thioredoxin, gp4, gp2.5, and gp1.7.

Host thioredoxin T7 polymerase by itself has a very low processivity. It dissociates from the primer-template after incorporating about 15 nucleotides. Upon infection of the host, T7 polymerase binds to host thioredoxin in 1:1 ratio. The hydrophobic interaction between thioredoxin and T7 polymerase helps to stabilize the binding of T7 polymerase to primer-template. In addition, the binding of thioredoxin increases T7 polymerase processivity to nearly 80-fold. The precise mechanism for how the thioredoxin-T7 polymerase complex is able to achieve such increase in processivity is still unknown. Binding of thioredoxin exposes a large number of basic amino acid residues in the thumb region of T7 polymerase. Several studies suggest that the electrostatic interaction between these positively charged basic residues with the negatively charged phosphate backbone of DNA and other accessory proteins is responsible for increased processivity in gp5/thioredoxin complex.

gp4

gp4 is a hexameric protein containing two functional domains: helicase domain and primase domain. The helicase domain unwinds double-stranded DNA to provide template for replication. The C-terminal tail of helicase domain contains several negatively charged acidic residues which make contact with the exposed basic residue of T7 polymerase/thioredoxin. These interactions help to load T7 polymerase/thioredoxin complex onto replication fork. The primase domain catalyzes the synthesis of short oligoribonucleotides. These oligoribonucleotides, called primers, are complementary to the template strand and used to initiate DNA replication. In T7 system, primase domain of one subunit interacts with primase domain of adjacent subunit. This interaction between primase domains acts as a brake to stop helicase when needed, which ensure the leading stand synthesis in-pace with lagging stand synthesis.

gp2.5

gp2.5 has similar function to single-stranded DNA binding protein. gp2.5 protects single-stranded DNA produced during replication and coordinates synthesis of leading and lagging strands through interaction between its acidic C-terminal tail and gp5/thioredoxin.

gp1.7

gp1.7 is a nucleoside monophosphate kinase, which catalyzes the conversion of deoxynucleoside 5'-monophosphates to di and triphosphate nucleotides, which accounts for the sensitivity of T7 polymerase to dideoxynucleotides (see Sequenase below).

Properties

… excerpt ends here. Continue reading the full article.

Illustrations

T7 DNA polymerase illustration
T7 DNA polymerase illustration
T7 DNA polymerase: Figure 3. Phosphoryl transfer catalyzed by T7 DNA polymerase. Two Mg2+ cations are used as ligands to form an octahedral network (red) and the amino acid residues in the enzyme active site are labelled in teal.
Figure 3. Phosphoryl transfer catalyzed by T7 DNA polymerase. Two Mg2+ cations are used as ligands to form an octahedral network (red) and the amino acid residues in the enzyme active site are labelled in teal.
T7 DNA polymerase: Figure 4. Phage T7 replication machinery. gp4 helicase domain unwinds double-stranded DNA into two single-stranded DNA template. Primase domain adds oligoribonucleotide primers. T7 polymerase/thioredoxin catalyzes synthesis of leading stand and lagging strand. gp2.5 coats the single-stranded DNA produced during replication. Interactions between T7 polymerase/thioredoxin and accessory proteins contributes to higher processivity in replisome assembly compared to gp5.
Figure 4. Phage T7 replication machinery. gp4 helicase domain unwinds double-stranded DNA into two single-stranded DNA template. Primase domain adds oligoribonucleotide primers. T7 polymerase/thioredoxin catalyzes synthesis of leading stand and lagging strand. gp2.5 coats the single-stranded DNA produced during replication. Interactions between T7 polymerase/thioredoxin and accessory proteins contributes to higher processivity in replisome assembly compared to gp5.

Worked examples

Example 1 — a first encounter with T7 DNA polymerase

Start with the simplest possible case. Write down what T7 DNA polymerase claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In chemistry, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to T7 DNA polymerase before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about T7 DNA polymerase ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of T7 DNA polymerase

In research
T7 DNA polymerase appears in chemistry research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses T7 DNA polymerase in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
T7 DNA polymerase is common in secondary-school and first-year university syllabi. It links to neighbouring topics DNA replication, EC 2.7.7, Phage proteins, so understanding it makes those chapters shorter.
In everyday life
Look for T7 DNA polymerase outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.
Ask Teacher Smith questions about this articleOpens your AI tutor with a question about “T7 DNA polymerase” →

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study T7 DNA polymerase in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what T7 DNA polymerase means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain T7 DNA polymerase out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is T7 DNA polymerase in simple terms?

T7 DNA polymerase is an enzyme used during the DNA replication of the T7 bacteriophage. During this process, the DNA polymerase “reads” existing DNA strands and creates two new strands that match the existing ones.

Why does T7 DNA polymerase matter?

Because it connects several chemistry ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study T7 DNA polymerase?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on T7 DNA polymerase.

Tags

  • DNA replication
  • EC 2.7.7
  • Phage proteins
  • T-phages

Keep exploring