Excinuclease

Excinuclease The excision nuclease involved in nucleotide exchange repair of DNA. 

Deoxyribonucleic acid footprinting studies have shown that HMG domains A and B inhibit cleavage by nucleases over a 12- to 15-base-pair region centered around the platinum adduct (81). The HMG proteins can modulate cisplatin cytotoxicity by inhibition of the excinuclease-mediated removal of Pt-d(GpG) adducts from DNA (82). However, this hypothesis has been questioned because there is no evidence for cellular protein shielding of Pt-d(GpG) adducts from repair enzymes (83). 

Xeroderma pigmentosum is caused by a defect in excision repair of thymine dimers, most freguently due to the absence of a UV-specific excinuclease, an enzyme that helps remove thymine dimers. 

The answer is C. Thymine dimers are repaired by the process of nucleotide excision repair, which involves many enzyme activities that recognize the mutated structure, cut the DNA strand on both sides of the mutation, remove (excise) the affected fragment, and then refill the gap. One of the major genes leading to xeroderma pigmentosoum encodes a specific excinuclease. 

FIGURE 25-24 Nucleotide-excision repair in E. coli and humans. The general pathway of nucleotide-excision repair is similar in all organisms. (J) An excinuclease binds to DNA at the site of a bulky lesion and cleaves the damaged DNA strand on either side of the lesion. (2) The DNA segment—of 13 nucleotides (13 mer) or 29 nucleotides (29 mer)—is removed with the aid of a helicase. (3) The gap is filled in by DNA polymerase, and the remaining nick is sealed with DNA ligase. 

The mechanism of eukaryotic excinucleases is quite similar to that of the bacterial enzyme, although 16 polypeptides with no similarity to the E. coli excinuclease subunits are required for the dual incision. As described in Chapter 26, some of the nucleotide-excision repair and base-excision repair in eukaryotes is closely tied to transcription. Genetic deficiencies in nucleotide-excision repair in humans give rise to a variety of serious diseases (Box 25-1). 

Encode ABC excinuclease subunits UvrA and UvrB Encode DNA polymerase V... 

In the rare genetic disease xeroderma pigmentosum, the cells cannot repair the damaged DNA, resulting in extensive accumulation of mutations and, consequently, skin cancers (Figure 29.28). The most common form of this disease is caused by the absence of the UV-specific excinuclease. 

DNA Repair. A connection between p53 and DNA repair was observed in p53-deficient cells that exhibited less global DNA repair [197-199] (but see [200]), as well as a reduced capacity to reactivate cisplatin- and UV-damaged reporter plasmids [173][201 ][202]. Furthermore, pretreatment with low levels of UV activated a protective response in which the levels of repair activity were elevated, an effect not observed in p53-deficient cells [202] [203]. It is possible that the p53 protein is directly involved in removing DNA damage since the protein recognizes both irradiated DNA and mismatches [ 162]. There is also evidence that p53 can interact with several components of the excinuclease, including RPA and the TFIIH-associated factors XPB and XPD [204] [205]. So far, however, there is no evidence to demonstrate a direct role for p53 in the nucleotide excision repair pathway. 

Ultraviolet light produces pyrimidine dimers in human DNA, as it does in E. coli DNA. Furthermore, the repair mechanisms are similar. Studies of skin fibroblasts from patients with xeroderma pigmentosum have revealed a biochemical defect in one form of this disease. In normal fibro-blasts, half the pyrimidine dimers produced by ultraviolet radiation are excised in less than 24 hours. In contrast, almost no dimers are excised in this time interval in fibroblasts derived from patients with xeroderma pigmentosum. The results of these studies show that xeroderma pigmentosum can be produced by a defect in the excinuclease that hydrolyzes the DNA backbone near a pyrimidine dimer. The drastic clinical consequences of this enzymatic defect emphasize the critical importance of DNA-repair processes. The disease can also be caused by mutations in eight other genes for DNA repair, which attests to the complexity of repair processes. 

Following ultraviolet damage of DNA in skin A specific excinuclease detects damaged areas... 

A specific enzyme excises the damaged portion (steps 2 and 3 can be combined if an excinuclease cuts on both sides of the damaged DNA). 

In E. coli, the key enzymatic complex is the ABC excinuclease, which has three subunits, UvrA... 

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