DNA polymerase III holoenzyme

Pol III can also refer to KNM Pol III, a Norwegian guard vessel from WW2

DNA polymerase III holoenzyme is the primary enzyme complex involved in prokaryotic DNA replication. The complex has high processivity (i.e. the number of nucleotides added per binding event) and, specifically referring to the replication of the E.coli genome, works in conjunction with four other DNA polymerases (Pol I, Pol II, Pol IV, and Pol V). Being the primary holoenzyme involved in replication activity, the DNA Pol III holoenzyme also has proofreading capabilities that correct replication mistakes by means of exonuclease activity working 3'->5'. DNA Pol III is a component of the replisome, which is located at the replication fork.

The replisome is composed of the following:

2 DNA Pol III enzymes, made up of α, ε and θ subunits.
- the α subunit has polymerization activity.
- the ε subunit has proofreading activity.
- the θ subunit stimulates the ε subunit's proofreading.
2 β units which act as sliding DNA clamps, they keep the polymerase bound to the DNA.
2 τ units which connect the 2 DNA Pol III enzymes.
1 γ unit 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.

Essentially, how it works is that after a single strand of DNA has a complimentary RNA primer synthesized from RNA polymerase II...

("!" for RNA, '"$" for DNA, "*" for polymerase)

> * * * * ! ! ! ! _ _ _ _ _ _ _ _ | RNA | <--ribose (sugar)-phosphate backbone G U A U | Pol | <--RNA primer * * * * |_II _ _| <--hydrogen bonding C A T A G C A T C C <--template ssDNA (single-stranded DNA) _ _ _ _ _ _ _ _ _ _ <--deoxyribose (sugar)-phosphate backbone $ $ $ $ $ $ $ $ $ $

It then "pushes off" the RNA polymerase II, and attaches where it previously was...

* * * * ! ! ! ! _ _ _ _ _ _ _ _ | DNA | <--ribose (sugar)-phosphate backbone G U A U | Pol | <--RNA primer * * * * |_III_ _| <--hydrogen bonding C A T A G C A T C C <--template ssDNA (single-stranded DNA) _ _ _ _ _ _ _ _ _ _ <--deoxyribose (sugar)-phosphate backbone $ $ $ $ $ $ $ $ $ $

And then the DNA polymerase III finishes off where the RNA polymerase II left off and "jumps off" just like what the RNA polymerase II did...

---> * * * * ! ! ! ! $ $ $ $ $ $ _ _ _ _ _ _ _ _ _ _ _ _ _ _| DNA | <--deoxyribose (sugar)-phosphate backbone G U A U C G T A G G| Pol | <--RNA primer * * * * * * * * * *|_III_ _| <--hydrogen bonding C A T A G C A T C C <--template ssDNA (single-stranded DNA) _ _ _ _ _ _ _ _ _ _ <--deoxyribose (sugar)-phosphate backbone $ $ $ $ $ $ $ $ $ $

However, after this, the RNA primer (denoted by exclamation marks) still has ribose as a sugar and uracil instead of thymine. These are replaced with the respective deoxyribose and thymine by DNA polymerase I (not shown). However, it is clear from the diagram that DNA polymerase III does much more "work" than DNA polymerase I, as DNA Pol III has to synthesize everything after the relatively tiny RNA primer, while DNA Pol I just modifies sections of the primer.

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See also