Biochemistry - The Chemical Reactions of Living Cells Volume 3 - D. Metzler 1980

Biochemical Genetics and the Synthesis of Nucleic Acids and Proteins
Mutations, Cancer, and Genetic Engineering
Repair of Damaged DNA

Maintaining a functioning copy of The Genome is essential for Cell survival. It is hardly surprising, therefore, that Cells possess entire sets of Enzymes that move along DNA Double helices, repair damage, and thereby help reduce mutation rates. The best-studied repair systems are those that "patch" Damage caused by ultraviolet radiation. Mutations that impair the repair capacity of E. coli and, consequently, increase sensitivity to UV irradiation are mapped at various loci on the chromosome map. The corresponding genes are designated as uvrA, B, C, D, F (identical to recF), and phr. Bacteria with mutant recA, B, C genes also exhibit heightened sensitivity to ultraviolet radiation, indicating that the recB, C Nucleases and the recA Gene product are involved not only in recombination but also in the repair of UV-induced damage.

One of the primary effects of ultraviolet irradiation on DNA is The formation of cyclobutane dimers (Ch. 13, Sec. D, 2) between adjacent pyrimidine rings located on the same DNA strand. To repair this damage, the dimer must be excised and replaced with new monomeric units. It has been shown that the uvrA and B genes determine the synthesis of Proteins that form a UV endonuclease [247], which is capable of introducing single-strand breaks on the 5' side of the dimer, liberating a 3'-OH group (Fig. 15-33). It is hypothesized that the UV endonuclease attack is followed by a 5'–3' exonucleolytic reaction that culminates in the excision of the cyclobutane dimer. This reaction can proceed either via the 5'–3' exonuclease activity of DNA polymerase I or with the participation of a specialized exonuclease. The resulting gap in one of the strands (Fig. 15-33) can subsequently be "patched" by DNA polymerase and ligase.

Another mechanism for repairing UV-induced damage involves photoreactivation by visible light or radiation from the near-ultraviolet spectrum (Ch. 13, Sec. C, 2). In bacteria, the phr gene encodes the synthesis of a specialized enzyme, DNA photolyase.

Repair systems that eliminate damage caused by ultraviolet radiation are not restricted to bacteria; they are found in All living organisms. Of particular interest is an autosomal recessive inherited disorder known as xeroderma pigmentosum. Individuals homozygous for the corresponding gene are exceptionally sensitive to ultraviolet rays and prone to developing multiple carcinomas. The disease is caused by several factors [248], one of which is a deficiency of a specific UV endonuclease. It has been shown that certain xeroderma cells lack photoreactivating capacity [249]. A third possible cause of xeroderma may be a deficiency in an endonuclease that specifically excises apurinic sites in DNA [249a].

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FIG. 15-33. 1. Excision of damage caused by ultraviolet irradiation.

Another human syndrome characterized by defective DNA Repair is progeria—a condition leading to premature Aging and numerous biochemical anomalies—as well as ataxia-telangiectasia. Skin fibroblasts from such patients cultured in vitro are incapable of repairing damage similar to that caused by X-rays [250, 250a], namely, DNA strand breaks induced by ⁶⁰Co gamma radiation. Fibroblasts from patients with ataxia-telangiectasia lack a functioning γ-endonuclease, the enzyme believed to initiate the Excision Repair of γ-modified bases. This condition differs from xeroderma, as cells from patients suffering from the latter disorder retain a normal ability to repair X-ray-induced damage.

Enzymes have also been identified that recognize mismatched Base Pairs in heteroduplexes, correct them [251], and remove bases modified by carcinogens from DNA [252].