Nucleotide Excision Repair

Nucleotide excision repair is a process whereby a damaged section of a DNA chain is cut out, or excised, followed by the action of first DNA polymerase and then DNA ligase to regenerate a covalently closed duplex at the site of the original damage.

The enzyme system involved, which in E. coli includes the products of the uvrA, uvrB, and uvrC genes, acts upon a number of DNA-damaged sistes containing lesions that may be quite bulky. Very similar systems exist in mammalian cells and in yeast.

As shown in Figure 25.12, the three-subunit UvrABC enzyme recognizes a lesion and, with the help of ATP hydrolysis, forces DNA to bend, leading to cleavage of the damaged strand at two sites--eight nucleotides to the 5' side of the damaged site and four or five nucleotides to the 3' side. The end result is a gap 12 or 13 nucleotides in length, with a 3' hydroxyl group and a 5' phosphate at the ends. Polymerase and ligase action then replaces the damaged 12-mer or 13-mer with undamaged DNA.

Helicase II, the product of the uvrD gene, is also required, presumably to unwind and remove the excised oligonucleotide, which is ultimately broken down by other enzymes. The UvrABC enzyme is not a classical endonuclease, because it cuts at two distinct sites, so the term excinuclease has been proposed for it, denoting its role in excision repair. This system also repairs DNA damage that results when two strands covalently crosslink to each other. In this case, the two strands are repaired sequentially (one after the other) in order to preserve an intact template strand.

A human excinuclease cleaves at positions -22 and +6 relative to a thymine dimer. A significant difference between this and the UvrABC enzyme is the involvement of two different endonucleases--one for cutting on the 5' side and one on the 3' side. The disease, xeroderma pigmentosum (XP), is actually a family of diseases, in which one or more enzymes of the excision pathway are deficient. The biological consequences of XP include extreme sensitivity to sunlight and a high incidence of skin cancers. In affected humans, there is at present no known way to treat the condition.

Recent studies of excision repair show that active genes (those undergoing transcription) are preferred substrates for excision repair, and within these genes the template DNA strand is preferentially repaired. Thus, the repair machinery is somehow directed toward sites where repair of DNA damage will do the most good. Transcription-coupled repair may initiate when a transcribing RNA polymerase stalls at the site of a DNA lesion. BRCA 1, a gene associated with increased risk of breast and ovarian cancer, has been implicated in transcription-coupled excision repair.

See also: Types and Consequences of DNA Damage, DNA-N-Glycosylases

INTERNET LINKS:

> 1. Nucleotide Excision Repair

> 2. Xeroderma Pigmentosum

> 3. Xeroderma Pigmentaosum Technical Notes

> 4. DNA Repair