DNA polymerase III holoenzyme
Primary enzyme complex involved in prokaryotic DNA replication
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Through Enzyme, Prokaryote and DNA replication
Components
The replisome is composed of the following:
- 2 DNA Pol III enzymes, each comprising α, ε and θ subunits. (It has been proven that there is a third copy of Pol III at the replisome.)
- the α subunit (encoded by the dnaE gene) has the polymerase activity.
- the ε subunit (dnaQ) has 3'→5' exonuclease activity.
- the θ subunit (holE) stimulates the ε subunit's proofreading.
- 2 β units (dnaN) which act as sliding DNA clamps, they keep the polymerase bound to the DNA.
- 2 τ units (dnaX) which act to dimerize two of the core enzymes (α, ε, and θ subunits).
- 1 γ unit (also dnaX) which acts as a clamp loader for the lagging strand Okazaki fragments, helping the two β subunits to form a unit and bind to DNA. The γ unit is made up of 5 γ subunits which include 3 γ subunits, 1 δ subunit (holA), and 1 δ' subunit (holB). The δ is involved in copying of the lagging strand.
- Χ (holC) and Ψ (holD) which form a 1:1 complex and bind to γ or τ. X can also mediate the switch from RNA primer to DNA.
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Activity
DNA polymerase III synthesizes base pairs at a rate of around 1000 nucleotides per second. DNA Pol III activity begins after strand separation at the origin of replication. Because DNA synthesis cannot start de novo, an RNA primer, complementary to part of the single-stranded DNA, is synthesized by primase (an RNA polymerase):
("!" for RNA, '"$" for DNA, "*" for polymerase)
--------> * * * * ! ! ! ! _ _ _ _ _ _ _ _ | <a rel="mw:WikiLink" href="./RNA" title="RNA" id="mwWQ">RNA</a> | <--<a rel="mw:WikiLink" href="./Ribose" title="Ribose" id="mwWg">ribose</a> (sugar)-phosphate backbone G U A U | <a rel="mw:WikiLink" href="./Polymerase" title="Polymerase" id="mwWw">Pol</a> | <--<a rel="mw:WikiLink" href="./RNA_primer" title="RNA primer" class="mw-redirect" id="mwXA">RNA primer</a> * * * * |_ _ _ _| <--hydrogen bonding C A T A G C A T C C <--<a rel="mw:WikiLink" href="./DNA" title="DNA" id="mwXQ">template</a> ss<a rel="mw:WikiLink" href="./DNA" title="DNA" id="mwXg">DNA</a> (single-stranded <a rel="mw:WikiLink" href="./DNA" title="DNA" id="mwXw">DNA</a>) _ _ _ _ _ _ _ _ _ _ <--<a rel="mw:WikiLink" href="./Deoxyribose" title="Deoxyribose" id="mwYA">deoxyribose</a> (sugar)-phosphate backbone $ $ $ $ $ $ $ $ $ $
Addition onto 3'OH
As replication progresses and the replisome moves forward, DNA polymerase III arrives at the RNA primer and begins replicating the DNA, adding onto the 3'OH of the primer:
* * * * ! ! ! ! _ _ _ _ _ _ _ _ | <a rel="mw:WikiLink" href="./DNA" title="DNA" id="mwZw">DNA</a> | <--<a rel="mw:WikiLink" href="./Deoxyribose" title="Deoxyribose" id="mwaA">deoxyribose</a> (sugar)-phosphate backbone G U A U | <a rel="mw:WikiLink" href="./Polymerase" title="Polymerase" id="mwaQ">Pol</a> | <--<a rel="mw:WikiLink" href="./RNA_primer" title="RNA primer" class="mw-redirect" id="mwag">RNA primer</a> * * * * |_III_ _| <--hydrogen bonding C A T A G C A T C C <--<a rel="mw:WikiLink" href="./DNA" title="DNA" id="mwaw">template</a> ss<a rel="mw:WikiLink" href="./DNA" title="DNA" id="mwbA">DNA</a> (single-stranded <a rel="mw:WikiLink" href="./DNA" title="DNA" id="mwbQ">DNA</a>) _ _ _ _ _ _ _ _ _ _ <--<a rel="mw:WikiLink" href="./Deoxyribose" title="Deoxyribose" id="mwbg">deoxyribose</a> (sugar)-phosphate backbone $ $ $ $ $ $ $ $ $ $
Synthesis of DNA
DNA polymerase III will then synthesize a continuous or discontinuous strand of DNA, depending if this is occurring on the leading or lagging strand (Okazaki fragment) of the DNA. DNA polymerase III has a high processivity and therefore, synthesizes DNA very quickly. This high processivity is due in part to the β-clamps that "hold" onto the DNA strands.
-----------> * * * * ! ! ! ! $ $ $ $ $ $ _ _ _ _ _ _ _ _ _ _ _ _ _ _| <a rel="mw:WikiLink" href="./DNA" title="DNA" id="mwdQ">DNA</a> | <--<a rel="mw:WikiLink" href="./Deoxyribose" title="Deoxyribose" id="mwdg">deoxyribose</a> (sugar)-phosphate backbone G U A U C G T A G G| <a rel="mw:WikiLink" href="./Polymerase" title="Polymerase" id="mwdw">Pol</a> | <--<a rel="mw:WikiLink" href="./RNA_primer" title="RNA primer" class="mw-redirect" id="mweA">RNA primer</a> * * * * * * * * * *|_III_ _| <--hydrogen bonding C A T A G C A T C C <--<a rel="mw:WikiLink" href="./DNA" title="DNA" id="mweQ">template</a> ss<a rel="mw:WikiLink" href="./DNA" title="DNA" id="mweg">DNA</a> (single-stranded <a rel="mw:WikiLink" href="./DNA" title="DNA" id="mwew">DNA</a>) _ _ _ _ _ _ _ _ _ _ <--<a rel="mw:WikiLink" href="./Deoxyribose" title="Deoxyribose" id="mwfA">deoxyribose</a> (sugar)-phosphate backbone $ $ $ $ $ $ $ $ $ $
Removal of primer
After replication of the desired region, the RNA primer is removed by DNA polymerase I via the process of nick translation. The removal of the RNA primer allows DNA ligase to ligate the DNA-DNA nick between the new fragment and the previous strand. DNA polymerase I & III, along with many other enzymes are all required for the high fidelity, high-processivity of DNA replication.