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Repair of abasic lesions, one of the most common types of damage found in DNA, is crucial to an organism's well-being. Studies in vitro indicate that after apurinic-apyrimidinic endonuclease cleaves immediately upstream of a baseless site, removal of the 5-terminal sugar-phosphate residue is achieved by deoxyribophosphodiesterase activity, an enzyme-mediated ␤-elimination reaction, or by endonucleolytic cleavage downstream of the baseless sugar. Synthesis and ligation complete repair.Eukaryotic RAD2 homolog 1 (RTH1) nuclease, by genetic and biochemical evidence, is involved in repair of modified DNA. Efficient endonucleolytic cleavage by RTH1 nuclease has been demonstrated for annealed primers that have unannealed 5-tails. In vivo, such substrate structures could result from repair-related strand displacement synthesis. Using 5-tailed substrates, we examined the ability of human RTH1 nuclease to efficiently remove 5-terminal abasic residues. A series of upstream primers were used to increasingly displace an otherwise annealed downstream primer containing a 5-terminal deoxyribose-5-phosphate. Until displacement of the first annealed nucleotide, substrates resisted cleavage. With further displacement, efficient cleavage occurred at the 3-end of the tail. Therefore, in combination with strand displacement activity, RTH1 nucleases may serve as an important alternative to other pathways in repair of abasic sites in DNA.Abasic lesions occur in DNA for several reasons, including spontaneous depurination (1), release of the base from a damaged sugar residue, or enzymatic removal of an inappropriate (e.g. uracil) or damaged (e.g. alkylated, deaminated, or oxidized) base by specialized glycosylases (2). Timely repair of abasic lesions is necessary, since during replication the lesion could lead to potentially lethal or mutagenic substitutions (3).Repair of an abasic site is predominantly initiated by an apurinic-apyrimidinic (AP) 1 endonuclease that cleaves immediately upstream of the baseless sugar, creating a 3Ј-hydroxyl terminus and a 5Ј-deoxyribose-5-phosphate terminus (4, 5). The next step, removal of the baseless sugar from the downstream strand, likely occurs by one of three distinct mechanisms.One mechanism involves removal of 5Ј-terminal sugar-phosphate residues by a Mg 2ϩ -dependent hydrolytic reaction, which releases 2-deoxyribose-5-phosphate. The activity responsible for this reaction, deoxyribophosphodiesterase (dRpase), was discovered in Escherichia coli (6). It was later shown to result from cleavage by either exonuclease I (7, 8) or the RecJ protein (9). The product of the RecJ gene was previously identified as a single strand-specific 5Ј-to 3Ј-exonuclease (10). Nonetheless, enzymes with dRpase activity have no associated double strand-specific 5Ј-to 3Ј-exonuclease function, so only a 1-nucleotide gap is produced after removal of the baseless sugar. A DNA polymerase fills in the single gap, and then a DNA ligase fuses the nick. There is support for this pathway, known as base excision repair. It has been s...

Two pathways for completion of DNA base excision repair (BER) have recently emerged. In one, called short patch BER, only the damaged nucleotide is replaced, whereas in the second, known as long patch BER, the monobasic lesion is removed along with additional downstream nucleotides. Flap endonuclease 1, which preferentially cleaves unannealed 5-flap structures in DNA, has been shown to play a crucial role in the long patch mode of repair. This nuclease will efficiently release 5-terminal abasic lesions as part of an intact oligonucleotide when cleavage is combined with strand displacement synthesis. Further gap filling and ligation complete repair. We reconstituted the final steps of long patch base excision repair in vitro using calf DNA polymerase ⑀ to provide strand displacement synthesis, human flap endonuclease 1, and human DNA ligase I. Replication protein A is an important constituent of the DNA replication machinery. It also has been shown to interact with an early component of base excision repair: uracil glycosylase. Here we show that human replication protein A greatly stimulates long patch base excision repair.To preserve an organism's genetic integrity, several DNA repair pathways have evolved. One pathway of particular importance is DNA base excision repair (BER), 1 because the types of lesions it addresses are among the most common. Typical lesions include alkylated, oxidized, or deaminated base moieties that are removed during the initiating step by a family of enzymes called DNA N-glycosylases (for review see Ref. 1). This enzyme family is subdivided into two categories, bifunctional and monofunctional, and each member is specific to certain monobasic lesions.Bifunctional glycosylases release not only the damaged base but also cleave the DNA backbone on the 3Ј side of the just formed baseless residue by utilizing an inherent AP lyase activity. In such instances, the 3Ј-terminal baseless sugar phosphate residue is further processed by a class II AP endonuclease, because this enzyme has both 3Ј-phosphodiesterase and 3Ј-phosphatase activities (for review see Ref.2). The resulting gap in the DNA is eventually filled, and the adjacent strands are then ligated to complete repair. Monofunctional glycosylases, however, release just the damaged base, creating a different type of DNA lesion: an abasic site. Internal abasic sites are also created by spontaneous release of purines under physiological conditions, so this newly created lesion is quite common, occurring an estimated 9000 -10,000 lesions/cell/day (3-5). These abasic sites are also handled by a class II AP endonuclease that cleaves on the 5Ј side of the baseless residue, creating a 3Ј terminus that supports primer extension (2). During the final steps of this pathway, the baseless sugar phosphate residue must be removed from the 5Ј terminus of the downstream strand so that both strands can be properly ligated.Two different mechanisms for removal of the abasic lesion in higher eukaryotes have recently emerged. In short patch base excision repair,...

This communication represents a consideration of non-visual interactions between environmental light and the eye. It covers the following topics: light in the environment; light entering the eye and transmission to the retina; observed fight damage; mechanisms of light damage; and considerations of the absorptive properties of artificial lenses. The intention of this paper is to summarize the available information and some of the current thinking about the non-visual interactions of the eye with short wavelength radiation. Many more experiments must be done for us to appreciate the full significance of this information for the benefit of man.