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Concept
DNA polymerase I
Summary
DNA polymerase I (or Pol I) is an enzyme that participates in the process of prokaryotic DNA replication. Discovered by Arthur Kornberg in 1956, it was the first known DNA polymerase (and the first known of any kind of polymerase). It was initially characterized in E. coli and is ubiquitous in prokaryotes. In E. coli and many other bacteria, the gene that encodes Pol I is known as polA. The E. coli Pol I enzyme is composed of 928 amino acids, and is an example of a processive enzyme — it can sequentially catalyze multiple polymerisation steps without releasing the single-stranded template. The physiological function of Pol I is mainly to support repair of damaged DNA, but it also contributes to connecting Okazaki fragments by deleting RNA primers and replacing the ribonucleotides with DNA.
In 1956, Arthur Kornberg and colleagues discovered Pol I by using Escherichia coli (E. coli) extracts to develop a DNA synthesis assay. The scientists added 14C-labeled thymidine so that a radioactive polymer of DNA, not RNA, could be retrieved. To initiate the purification of DNA polymerase, the researchers added streptomycin sulfate to the E. coli extract. This separated the extract into a nucleic acid-free supernatant (S-fraction) and nucleic acid-containing precipitate (P-fraction). The P-fraction also contained Pol I and heat-stable factors essential for the DNA synthesis reactions. These factors were identified as nucleoside triphosphates, the building blocks of nucleic acids. The S-fraction contained multiple deoxynucleoside kinases. In 1959, the Nobel Prize in Physiology or Medicine was awarded to Arthur Kornberg and Severo Ochoa "for their discovery of the mechanisms involved in the biological synthesis of Ribonucleic acid and Deoxyribonucleic Acid."
Pol I mainly functions in the repair of damaged DNA. Structurally, Pol I is a member of the alpha/beta protein superfamily, which encompasses proteins in which α-helices and β-strands occur in irregular sequences. E. coli DNA Pol I consists of multiple domains with three distinct enzymatic activities.
Official source
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