Arabidopsis UVH3 gene is a homolog of the Saccharomyces cerevisiae RAD2 and human XPG DNA repair genes

To identify mechanisms of DNA repair in Arabidopsis thaliana, we have analy zed a mutant (uvh3) which exhibits increased sensitivity to ultraviolet (UV ) light, H2O2 and ionizing radiation and displays a premature senescence ph enotype. The uvh3 locus was mapped within chromosome III to the GL1 locus. A cosmid contig of the GL1 region was constructed, and individual cosmids w ere used to transform uvh3 mutant plants. Cosmid N9 was found to confer UV- resistance, H2O2-resistance and a normal senescence phenotype following tra nsformation, indicating that the UVH3 gene is located on this cosmid and th at all three phenotypes are due to the same mutation. Analysis of cosmid N9 sequences identified a gene showing strong similarity to two homologous re pair genes, RAD2 (Saccharomyces cerevisiae) and XPG (human), which encode a n endonuclease required for nucleotide excision repair of UV-damage. The uv h3 mutant was shown to carry a nonsense mutation in the coding region of th e AtRAD2/XPG gene, thus revealing that the UVH3 gene encodes the AtRAD2/XPG gene product. In humans, the homologous XPG protein is also involved in re moval of oxygen-damaged nucleotides by base excision repair. We discuss the possibility that the increased sensitivity of the uvh3 mutant to H2O2 and the premature senescence phenotype might result from failure to repair oxyg en damage in plant tissues. Finally, we show that the AtRAD2/XPG gene is ex pressed at moderate levels in all plant tissues.