8.11
Reparo por Excisão de Nucleotídeos
Distorção e Danos ao DNA
As células são regularmente expostas a mutagênicos – fatores ambientais que podem danificar o DNA e gerar mutações. A radiaçã…
O reparo por excisão de nucleotídeos, ou NER, corrige lesões volumosas de DNA causadas por radiação UV e carcinógenos químicos.
Essas lesões volumosas, como dímeros de pirimidina, distorcem a hélice do DNA e interferem na replicação e transcrição do DNA.
Em E. coli, as proteínas de reparo UvrA e UvrB formam um complexo que escaneia o DNA genômico em busca de lesões estruturais.
Uma vez que encontra uma lesão, o UvrA se dissocia, fazendo com que o UvrB se ligue firmemente ao DNA distorcido.
O UvrB ligado ao DNA então recruta UvrC, uma endonuclease, que faz uma incisão em ambos os lados da lesão.
Em seguida, a helicase UvrD desenrola o DNA e excisa o fragmento que carrega a lesão.
Após a excisão, a DNA polimerase preenche a lacuna com novos nucleotídeos complementares, e a enzima DNA ligase sela a lacuna entre o DNA novo e o antigo, completando o reparo.
Alternativamente, se uma RNA polimerase de transcrição ativa encontrar a lesão volumosa, ela pode recrutar diretamente o UvrB para o local da lesão.
Isso inicia o reparo por excisão de nucleotídeos acoplado à transcrição que ocorre da mesma maneira que o NER regular para completar o reparo e continuar a transcrição.
Q1: What types of DNA damage does nucleotide excision repair fix?
Nucleotide excision repair fixes bulky DNA lesions caused by UV radiation and chemical carcinogens, such as pyrimidine dimers. These lesions distort the DNA helix and interfere with DNA replication and transcription. If left unrepaired, they can cause mutations leading to cancer or disease depending on which DNA sequences are disrupted.
Q2: How do UvrA and UvrB proteins identify damaged DNA in prokaryotes?
In E. coli, UvrA and UvrB form a complex that scans genomic DNA for structural lesions. When the complex encounters a lesion, UvrA dissociates and UvrB binds tightly to the distorted DNA region. This recognition of physical aberrations allows the repair machinery to flag the damaged area for removal and repair.
Q3: What is the role of UvrC endonuclease in nucleotide excision repair?
UvrC is an endonuclease recruited by DNA-bound UvrB that makes incisions on either side of the lesion. These cuts allow the damaged DNA fragment to be excised from the strand. Following excision, DNA polymerase fills the gap with new complementary nucleotides, and DNA ligase seals the repair.
Q4: How does transcription-coupled nucleotide excision repair differ from regular NER?
Transcription-coupled NER is initiated when an actively transcribing RNA polymerase encounters a bulky lesion and directly recruits UvrB to the damage site. This alternative pathway proceeds identically to regular NER but is triggered by transcription blockage rather than routine DNA scanning, allowing repair and transcription to resume.
Q5: Why do eukaryotes require more proteins than prokaryotes for nucleotide excision repair?
Unlike prokaryotes, which use three proteins—UvrA, UvrB, and UvrC—eukaryotes employ more than a dozen proteins to regulate nucleotide excision repair. This increased complexity reflects the greater size and organization of eukaryotic genomes and the need for more sophisticated damage recognition and coordination mechanisms.
Q6: What happens when nucleotide excision repair genes are mutated in humans?
Mutations in the NER pathway cause diseases such as Xeroderma pigmentosum, associated with a 2000-fold increase in skin cancer incidence. XP patients are highly sensitive to UV exposure, developing severe burns after brief sunlight exposure, and often show premature aging and neurological abnormalities. Without functional repair, DNA damage accumulates, leading to abnormal cell death or cancerous tumors.
Q7: What is the final step after DNA polymerase fills the gap in nucleotide excision repair?
After DNA polymerase fills the gap with new complementary nucleotides, the enzyme DNA ligase seals the gap between the new and old DNA strands. This ligation completes the nucleotide excision repair process and restores the integrity of the DNA molecule.