8.11:
Nucleotide Excision Repair
Nucleotide excision repair, or NER, fixes bulky DNA lesions caused by UV radiation and chemical carcinogens.
These bulky lesions, such as pyrimidine dimers, distort the DNA helix and interfere with DNA replication and transcription.
In E. coli, the repair proteins UvrA and UvrB form a complex that scans the genomic DNA for structural lesions.
Once it encounters a lesion, the UvrA dissociates, causing UvrB to bind tightly to the distorted DNA.
The DNA-bound UvrB then recruits UvrC, an endonuclease, which makes an incision on either side of the lesion.
Next, the UvrD helicase unwinds the DNA and excises the fragment carrying the lesion.
Following excision, DNA polymerase fills the gap with new complementary nucleotides, and the enzyme DNA ligase seals the gap between the new and old DNA, completing the repair.
Alternately, if an actively transcribing RNA polymerase encounters the bulky lesion, it can directly recruit the UvrB to the site of the lesion.
This initiates transcription-coupled nucleotide excision repair that proceeds in the same manner as regular NER to complete the repair and continue transcription.
8.11:
Nucleotide Excision Repair
DNA Distortion and Damage
Cells are regularly exposed to mutagens—factors in the environment that can damage DNA and generate mutations. UV radiation is one of the most common mutagens and is estimated to introduce a significant number of changes in DNA. These include bends or kinks in the structure, which can block DNA replication or transcription. If these errors are not fixed, the damage can cause mutations, which in turn can result in cancer or disease depending on which sequences are disrupted.
Identification and Repair of Damaged Regions
Nucleotide excision repair relies on specific protein complexes to recognize damaged regions of DNA and flag them for removal and repair. In prokaryotes, the process involves three proteins—UvrA, UvrB, and UvrC. The first two proteins work together as a complex, traveling along the DNA strands to detect any physical aberrations.
Once identified, the strands at the damaged location are separated, and endonuclease enzymes such as UvrC cut and excise the affected region. DNA polymerase then fills the gap with new nucleotides, and the enzyme DNA ligase seals the edges between the new and old DNA. Unlike prokaryotes, more than a dozen proteins operate to regulate DNA repair in eukaryotes.
Mutations in NER Pathway
In humans, mutations in the NER pathway can cause diseases such as Xeroderma pigmentosum, which is associated with a 2000-fold increase in the incidence of skin cancer. Individuals suffering from XP are highly sensitive to UV exposure and can develop severe skin burns after just a few minutes of exposure to sunlight. Additionally, XP patients can show signs of premature aging and often develop neurological abnormalities. Without a properly working repair mechanism, DNA damage can accumulate and lead to abnormal cell death or potentially cancerous tumors.