DNA polymerase I
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[[image:PolymeraseDomains.jpg|right|thumb|450px|<center>Functional domains in the Klenow Fragment (left) and DNA Polymerase I (right).</center>]]
'''DNA Polymerase I''' (or '''Pol I''') is an [[enzyme]] that participates in the process of [[DNA replication]] in [[prokaryote]]s. It is composed of 928 amino acids, and is an example of a [[Processivity|processive]] enzyme - it can sequentially catalyze multiple polymerisations. Discovered by [[Arthur Kornberg]] in [[1956]]<ref>{{cite journal
|title=Enzymatic Synthesis of Deoxyribonucleic Acid. I. Preparation of Substrates and Partial Purification of an Enzyme from ''Escherichia coli'' |author=Lehman, I. R.
|coauthors= Bessman, M. J.; Simms, E. S.; Kornberg, A.
|year=1958
|month=Jul
|journal=J. Biol. Chem.
|volume=233
|issue=1
|pages=163–170
|pmid=13563462
}}</ref>, it was the first known [[DNA polymerase]] (and, indeed, the first known [[polymerase]] of any kind). It was initially characterized in ''[[E. coli]]'', although it is ubiquitous in [[prokaryote]]s. In ''E. coli'' and many other bacteria, the [[gene]] which encodes Pol I is known as '''''polA'''''.
Pol I possesses three enzymatic activities:
# A 5' -> 3' (forward) DNA polymerase activity, requiring a 3' [[Primer (molecular biology)|primer]] site and a template strand
# A 3' -> 5' (reverse) [[exonuclease]] activity that mediates [[proofreading]]
# A 5' -> 3' (forward) exonuclease activity mediating [[nick translation]] during [[DNA repair]].
In the replication process, DNA Polymerase I removes the [[RNA]] primer (created by [[Primase]]) from the [[lagging strand]] and fills in the necessary [[nucleotides]] of the [[Okazaki fragments]] (see ''[[DNA replication]]'') in 5' -> 3' direction, proofreading for mistakes as it goes. It is a template-dependent enzyme - it only adds nucleotides that correctly [[base pair]] with an existing DNA strand acting as a template. [[Ligase]] then joins the various fragments together into a continuous strand of [[DNA]].
Despite its early characterisation, it quickly became apparent that Pol I was not the enzyme responsible for most DNA synthesis — DNA replication in ''E. coli'' proceeds at approximately 1,000 nucleotides/second, while the rate of synthesis by pol I averages only 20 nucleotides/second. Moreover, its cellular abundance of approximately 400 molecules per cell did not correlate with the fact that there are typically only two [[replication fork]]s in ''E. coli''. Moreover, it is insufficiently [[Processivity|processive]] to copy an entire [[genome]], as it falls off after incorporating only 25-50 [[nucleotides]]. Its role in replication was proven when, in 1969, [[John Cairns]] isolated a viable pol I [[mutant]] that lacked the polymerase activity<ref>{{cite journal
|journal=Nature
|volume=224
|issue=5225
|pages=1164–66
|month=Dec
|year=1969
|doi=10.1038/2241164a0
|pmid=4902142
|title=Isolation of an E. coli Strain with a Mutation affecting DNA Polymerase
|author=Paula de Lucia
|coauthors=John Cairns
}}</ref>. Cairns' lab assistant Paula De Lucia created thousands of cell free extracts from E.coli colonies and assayed them for DNA-polymerase activity. The 3,478th clone contained the [[polA]] mutant, which was named by Cairns to credit "Paula" [De Lucia]<ref>{{cite journal
|journal=Nature Reviews Molecular Cell Biology
|volume=7
|issue=2
|pages=143–7
|month=Feb
|year=2006
|doi=10.1038/nrm1787
|pmid=16493419
|title=Timeline: The eureka enzyme: the discovery of DNA polymerase
|author=Errol C. Friedberg
}}</ref>. It was not until the discovery of [[DNA polymerase III]] that the main replicative DNA polymerase was finally identified.
== Research applications ==
DNA polymerase I obtained from ''E. coli'' is used extensively for [[molecular biology]] research. However, the 5' -> 3' exonuclease activity makes it unsuitable for many applications. Fortunately this undesirable enzymatic activity can be simply removed from the holoenzyme to leave a useful molecule called the [[Klenow fragment]], widely used in [[molecular biology]]. Exposure of DNA polymerase I to the protease [[subtilisin]] cleaves the molecule into a smaller fragment, which retains only the DNA polymerase and proofreading activities.
== See also ==
*[[DNA polymerase II]]
*[[DNA polymerase III]]
==References==
{{Reflist}}
{{transferase-stub}}
{{Polymerases}}
[[category:EC 2.7.7]]
[[Category:DNA replication]]
[[Category:Enzymes]]
[[it:DNA polimerasi I]]