What is DNA polymerase III

DNA polymerase III

Synonym: Pol III
English: DNA polymerase III

1 definition

The DNA polymerase III is the replicative polymerase of prokaryotes. It is a DNA-dependent DNA polymerase with high processivity and low error rate.

2 structure

The holoenzyme of DNA polymerase III is a large protein complex consisting of ten different subunits and a molecular weight of 900 kDA. The core enzyme is formed by the three subunits α, ε and θ. The subunits γ, δ, δ ', χ, Ψ together form the γ complex, which carries out the loading of the DNA with the polymerase (English: "clamp loader"). Some subunits are duplicated.

αdnaAcatalytic subunit of polymerase
εdnaQ3'-5 'exonuclease
θget connects the proteins of the core enzyme
γdnaX clamp loader
δholA clamp loader
δ ' getB clamp loader
χholCclamp loader
ψholD clamp loader
βdnaN forms a bracket and prevents dissociation from the DNA.
τdnaX connects two core complexes to form a dimer

3 biochemistry

The DNA polymerase III is responsible for the polymerisation of the DNA in the 5'-3 'direction. This catalytic function can be performed by the α-subunit alone. However, the processivity of the isolated α-subunit is very low, with fewer than ten base pairs synthesized in a row. Only the associated factors increase processivity. In the experiment, up to 50,000 base pairs could be catalyzed without interruption.

DNA polymerase III also has 3'-5 'exonuclease activity and thus the ability to proofread. As soon as the wrong nucleotide has been inserted, the replication stops briefly and removes it. The polymerase then synthesizes the correct nucleotide and continues DNA synthesis.

4 function

DNA polymerase III is responsible for elongating DNA synthesis during replication. After opening the DNA at the origin of replication by DnaA and recruiting the helicases DnaB / DnaC, the enzyme primase (DnaG) synthesizes a short RNA primer. The DNA polymerase III extends this on the leading strand and the following strand. Since the synthesis only takes place in the 5'-3 'direction, it is not continuous on the next strand, which results in the Okazaki fragments. These have a length of approx. 1,000-2,000 bp. DNA polymerase I replaces the RNA primers with DNA.

The polymerase III is much rarer in the prokaryotic cell with approx. 15 copies than the DNA polymerase I with approx. 400.

see also:Replication

5 literature

  • Anthony J.F. Griffiths: Introduction to Genetic Analysis. W.H. Freeman and Company, 2005, pp. 242-245.
  • Klug, W.S. Concepts of Genetics. 10th edn. Pearson Education (2012), 276-277.