UvrABC endonuclease is a multienzyme complex in E.coli

Escherichia coli

Escherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...

 bacteria involved in DNA repair

DNA repair

DNA repair refers to a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as UV light and radiation can cause DNA damage, resulting in as many as 1...

 mechanism by nucleotide excision repair

Nucleotide excision repair

Nucleotide excision repair is a DNA repair mechanism. DNA constantly requires repair due to damage that can occur to bases from a vast variety of sources including chemicals, radiation and other mutagens...

, and it is, therefore, sometimes called an excinuclease

Excinuclease

Excision endonuclease, also known as excinuclease or UV-Specific Endonuclease, is a nuclease which excises a fragment of nucleotides during DNA repair. The excinuclease cuts out a fragment by hydrolyzing two phosphodiester bonds, one on either side of the lesion in the DNA...

. This UvrABC repair process, sometimes called the short-patch process, involves the removal of 12 nucleotides where a genetic mutation has occurred followed by a DNA polymerase, replacing these aberrant nucleotides with the correct nucleotides and completing the DNA repair

DNA repair

DNA repair refers to a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as UV light and radiation can cause DNA damage, resulting in as many as 1...

. This enzyme complex is able to repair many different types of damage, including cyclobutyl dimer formation.

Mechanism

1. Two UvrA proteins form a dimer and they both have ATPase/GTPase activity.
2. The UvrA dimer
   Protein dimer
   In biochemistry, a dimer is a macromolecular complex formed by two, usually non-covalently bound, macromolecules like proteins or nucleic acids...
   
    binds with UvrB and forms a trimer that is able to detect DNA damage.The UvrA dimer functions as the unit responsible for the detection of DNA damage, probably through a mechanism of detecting distortions in the DNA double helix.
3. The UvrB part of the trimer attaches to the double helix at the damaged site.
4. The UvrA dimer leaves and an UvrC protein comes in and binds to the UvrB monomer
   Monomer
   A monomer is an atom or a small molecule that may bind chemically to other monomers to form a polymer; the term "monomeric protein" may also be used to describe one of the proteins making up a multiprotein complex...
   
    and, hence, forms a new UvrBC dimer.
5. This dimer is responsible for cleaving the nucleotides either side of the DNA damage. UvrB cleaves a phosphodiester bond
   Phosphodiester bond
   A phosphodiester bond is a group of strong covalent bonds between a phosphate group and two 5-carbon ring carbohydrates over two ester bonds. Phosphodiester bonds are central to all known life, as they make up the backbone of each helical strand of DNA...
   
    4 nucleotides downstream of the DNA damage, and the UvrC cleaves a phosphodiester bond
   Phosphodiester bond
   A phosphodiester bond is a group of strong covalent bonds between a phosphate group and two 5-carbon ring carbohydrates over two ester bonds. Phosphodiester bonds are central to all known life, as they make up the backbone of each helical strand of DNA...
   
    8 nucleotides upstream of the DNA damage and created 12 nucleotide excised segment.
6. DNA helicase II (sometimes called UvrD) then comes in and removes the excised segment by removing the base pairing. The UvrB still remain in place even though UvrC has disassociated at this stage, as UvrB may be involved to prevent the reannealing
   Reannealing
   Reannealing is the process by which two single strands of DNA combine to form double-stranded DNA. The term is most commonly used in its application in DNA amplification, the process by which DNA is copied in the lab....
   
    of the excised DNA.
7. DNA polymerase I
   DNA polymerase I
   DNA Polymerase I is an enzyme that participates in the process of DNA replication in prokaryotes. It is composed of 928 amino acids, and is an example of a processive enzyme - it can sequentially catalyze multiple polymerisations. Discovered by Arthur Kornberg in 1956, it was the first known...
   
   comes in and fills in the correct nucleotides sequence, kicking off UvrB in the process, and the last phosphodiester bond is completed by DNA ligase.