Structure and Properties of DNA Photoproducts

Exposure of DNA to light leads to a variety of types of DNA damage, some that result from direct absorption of light by the DNA, and others that result from indirect (photosensitized) pathways involving absorption of light by other molecules that then cause damage. While many photoproducts are stable on repair and replication timescales, others are converted to other products, either spontaneously, or following absorption of light. The photochemistry of bases, dinucleotides, oligonucleotides, and DNA has been extensively reviewed in a number monographs (1-4) and review articles (5-7). Because of the ubiquitous nature of DNA photoproducts, many organisms have evolved specific enzymes for their repair, and most, if not all, photoproducts are also subject to repair by general excision repair systems. These include the base excision repair (BER) cis-syn dimer-specific glycosylases (8), the cis-syn and photoproduct-specific photolyases (9,10), the spore photoproduct-specific lyase (11), and the nucleotide excision repair (NER) systems such as the Escherichia coli uvrABC and human excinuclease (12,13), as well as the UVDE-initiated alternate excision repair (AER) system found in yeast (14,15). This chapter will focus on the structure and properties of photoproducts that result from direct absorption of light by DNA and their secondary products, and how some of these features may contribute to their recognition by repair enzymes. Discussions of photosensitized reactions of DNA can be found elsewhere (5,7).