Nucleotide Excision Repair in Yeast: Recent Progress and Implications

Abstract

Nucleotide excision repair (NER) is a ubiquitous process by which damaged bases are excised from the genome of living cells as oligonucleotide fragments (reviewed in Friedberg et al. 1995). Recent years have witnessed significant progress in our understanding of the biochemistry of NER in the yeast Saccharomyces cerevisiae (reviewed in Prakash et al. 1993; Friedberg et al. 1995; Friedberg 1996a). The establishment of a cell-free system that monitors NER of damaged plasmid DNA in vitro (Wang et al. 1995a, 1996) has facilitated the systematic screening of yeast strains carrying mutations in multiple genes known to be required for or involved in NER. Additionally, the polypeptides encoded by many of these genes have been purified and the early steps in the NER process have been reconstituted in vitro (Guzder et al. 1995a). At least 26 gene products are believed to be required for or involved in NER (Table 1). The Rad1/Rad10 heterodimeric complex and the Rad2 protein are now known to function as junction-specific endonucleases which specifically cleave DNA at single-strand/duplex junctions with opposite singlestrand polarity (Habraken et al. 1993; Bardwell et al. 1994; Fig. 1). Incisions (nicks) generated by these two endonucleases result in the formation of oligonucleotide fragments ~24–27 nucleotides in size, which include damaged bases (Guzder et al. 1995a). Displacement of the oligonucleotides generates single-strand gaps that are filled in by repair synthesis and sealed by ligation.