Enzymatic photoreactivation

Adjacent pyrimidine bases in a DNA strand form dimers with high efficiency after absorbing ultraviolet light (fig. 26.16). By contrast, purines are quite resistant to damage by ultraviolet. Pyrimidine dimers formed within an otherwise intact DNA duplex have provided a useful substrate to assay for DNA repair. These dimers can be repaired directly by enzymatic photoreactivation (fig. 26.17). The... 

Thymine dimers may be monomerized from DNA by enzymatic photoreactivation. In this case no nucleotides are removed in the repair reaction. 

Hedrick WR, Webb MD, Zimbrick JD (1982) Spin trapping of reactive uracilyl radicals produced by ionizing radiation in aqueous solutions. Int J Radiat Biol 41 435-442 Heelis PF, Deeble DJ, Kim S-T (1992) Splitting of cis-syn cyclobutane thymine-thymine dimers by radiolysis and its relevance to enzymatic photoreactivation. Int J Radiat Biol 62 137-143 Hems G, Eidinoff ML (1958) Effect of X-irradiation on aqueous solutions of adenosine diphosphate. Radiat Res 9 305-311... 

No. Dimers can also be repaired directly by enzymatic photoreactivation. The photoreactivating enzyme binds to the DNA in the region of the dimer to form a complex that absorbs visible light and catalyzes cleavage of the covalent linkage between the components of the dimer. Photoreactivation occurs in both bacterial and mammalian cells. 

Repair of DNA damage including reversal of damage (e.g. enzymatic photoreactivation, alkylated nucleotide repair and ligation of strand breaks) and excision repair and... 

The book comprises eleven chapters contributed by experts in the field. It opens with discussions of the primary modes of the repair of DNA base damage in the strict biochemical sense, namely, base excision repair (BER), enzymatic photoreactivation (EPR) of both cyclobutane pyrimidine dimers and [6-4] photoproducts, nucleotide excision repair (NER), mismatch repair (MMR), and the repair of DNA double-strand breaks (DSBR). 

Experiments were carried out which showed that the ratio of TT/UT (4 8) observed in the acid hydrolysate of a DNA was nearly the same as the ratio (5 2) found in the products of the more gentle enzymatic hydrolysis. Other experiments showed that the photoreactivating enzyme could excise all three dimers equally readily, that all three dimers were implicated in the photoinactivation of primer DNA, and that CT dimers were excised from the DNA of radiation-resistant bacteria. 

Photoreactivation is a light-induced (300-600 nm) enzymatic cleavage of a thymine dimer to yield two thymine monomers. It is accomplished by photolyase, an enzyme that acts on dimers contained in single- and double-stranded DNA. 

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