The UvrABC Endonuclease (Excinuclease) Repair of DNA in Prokaryotes
What is Nucleotide Excision Repair (NER)?
As the name suggests it is a type of DNA repair mechanism. In nucleotide excision repair or NER, the damaged base along with a short stretch of healthy strand is removed and later the gap is refilled with correct nucleotides. Thus the NER pathway operates by ‘cut and patch’ mechanism. Even though nucleotide excision repair mechanism is present in prokaryotes and eukaryotes, the components of the pathways in both groups shows considerable variations. The prokaryotes shows relatively simple nucleotide excision repair mechanism, where as in eukaryotes, the NER pathway is quite complex with many enzymes.
Image source: wikipedia
Nucleotide Excision Repair can repair a variety of bulky lesions of the DNA such as pyrimidine dimers formed by UV irradiation and it can also remove chemically modified bases.
Classification of NER:
(1). UvrABC Endonuclease NER in Prokaryotes
(2). NER in Eukaryotes
There are two types of NER mechanism in eukaryotic cells
(b). Global Genomic Nucleotide Excision Repair (GG-NER)
In this post, we will discuss only the NER mechanism of prokaryotes. The NER mechanism in prokaryotes is better known as UvrABC Endonuclease repair.
Enzymes involved in NER pathway of prokaryotes:
Nucleotide excision repair pathway in prokaryotes is orchestrated by UvrABC endonuclease complex. The NER method is also assisted by DNA polymerase I and DNA ligase enzymes. UvrABC endonuclease complex is a special class of endonuclease enzyme involved in DNA repair of prokaryotes. The name is derived from the root term ultraviolet radiation, since the level of most of these enzymes will be elevated in the bacterial cells when the cells are exposed to UV light.
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What is Base Excision Repair or BER?
As the name suggests, it is a type of DNA repair mechanism present in both prokaryotes and eukaryotes. In this DNA repair method, the damaged or unnatural base in the DNA double helix is removed by cleaving the N-glycosyl bond without disrupting the phosphodiester bond. N-glycosyl bond is the covalent bond which connects the nitrogen base with the deoxy-ribose sugar of the DNA. The importance of this bond is that, if an enzyme can cleave this particular bond, as it happens during base excision repair, it can selectively excise the nitrogen base from the DNA without altering the phosphodiester backbone. The main difference of Base Excision Repair from other repair mechanisms is that here only the damaged base is excised from the DNA strand, the phosphodiester back bone is not disturbed for the removal or the damaged bases. But in other DNA repair mechanisms such as mismatch repair or nucleotide excision repair, the damaged nucleotide (nucleotide = nitrogen base + sugar + phosphate group) as such is removed first and refilled by with correct nucleotides. Dear students, please remember, the cleavage of phosphate back bone is also occurs here but it happens in the second stage, not as the part of the removal of nitrogen base.
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connections between different observations.
Stephen W. Hawking, 2011
What is Photoreactivation?
Photoreactivation is a type of DNA repair mechanism present in prokaryotes, archaea and in many eukaryotes. It is the recovery of ultraviolet irradiated damages of DNA by visible light. As the name suggests, it is a light dependent process. In this DNA repair method cells recovers its DNA after UV exposure induced damages. The UV light is lethal to cellular DNA since it induces structural lesions in the DNA by the formation of pyrimidine dimer. Photoreactivation process removes the pyrimidine dimers without altering other nucleotides in the DNA by using energy obtained from visible light. Photoreactivation is the first discovered DNA repair mechanism in the cell. Major credits for the discovery of Photoreactivation goes to Professor Aziz Sancar for which he was shared the 2015 Nobel Prize in chemistry along with Thomas Lindhal and Paul Modrich.