= Bacteriophage T4 DNA polymerase
= T4 DNA polymerase
{c}
{synonym}
\b[Bacteriophage T4 DNA polymerase] is a <DNA polymerase> associated with <bacteriophage T4>. It extends an existing paired strand end in the $5'\to3'$ direction using a <DNA> template and <deoxynucleoside triphosphates>. A <primer (molecular biology)> supplies the extendable end. The enzyme also has $3'\to5'$ <exonuclease> activity, supporting <DNA polymerase proofreading>; the characterized enzyme lacks a $5'\to3'$ exonuclease function.<T4 DNA Polymerase by New England Biolabs>[Product Information: synthesis direction, template/primer requirement and exonuclease activities]{ref}
Extension and removal are competing chemical outcomes, not contradictory strand directions. A simplified kinetic example assigns rates $k_{\rm ext}$ and $k_{\rm exo}$ to two possible exits from one enzyme–strand-end state. If both exits have exponential waiting times, the probability that extension happens first is
$$
P_{\rm ext}=\frac{k_{\rm ext}}{k_{\rm ext}+k_{\rm exo}}.
$$
Stipulated rates $100$ and $1\,\mathrm{s^{-1}}$ give $P_{\rm ext}\approx0.990$; rates $0.1$ and $10\,\mathrm{s^{-1}}$ give approximately $0.0099$. These illustrative numbers show how slowing extension relative to removal can favor proofreading. They are not measured constants for this enzyme or a complete model of its active-site transitions.
The same protein also supplied a historical binding target. In their 1990 primary study, <C. Tuerk> and <L. Gold> selected <RNA> sequences that bound T4 DNA polymerase, finding distinct selected sequences with equivalent binding constants.<Systematic evolution of ligands by exponential enrichment: RNA ligands to bacteriophage T4 DNA polymerase by C. Tuerk and L. Gold>[Abstract: eight-base variant region and two selected RNA binders]{ref} Measuring binding to a protein and measuring its catalytic DNA synthesis answer different experimental questions. The selected RNA is an <aptamer>; it is not itself the polymerase.
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