SummaryAtercc1, the recently characterized Arabidopsis homologue of the Ercc1 (Rad10) protein, is a key component of nucleotide excision repair as part of a structure-specific endonuclease which cleaves 5¢ to UV photoproducts in DNA. This endonuclease also acts in removing overhanging non-homologous DNA 'tails' in synapsed recombination intermediates. We have previously demonstrated this recombination function of the Arabidopsis thaliana Xpf homologue, AtRad1p, and show here that recombination between plasmid DNA substrates containing non-homologous tails is specifically reduced 12-fold in atercc1 mutant plants compared with the wild type. Furthermore, using chromosomal tandem-repeat recombination substrates, we show that AtErcc1p is required for bleomycin induction of mitotic recombination in the chromosomal context. This work thus confirms both the specific and general recombination roles of the Atercc1 protein in recombination in Arabidopsis.

Using a specific recombination assay, we show in the plant Arabidopsis thaliana that AtRad1 protein plays a role in the removal of non-homologous tails in homologous recombination. Recombination in the presence of non-homologous overhangs is reduced 11-fold in the atrad1 mutant compared with the wild-type plants. AtRad1p is the A. thaliana homologue of the human Xpf and Saccharomyces cerevisiae Rad1 proteins. Rad1p is a subunit of the Rad1p/Rad10p structurespecific endonuclease that acts in nucleotide excision repair and inter-strand crosslink repair. This endonuclease also plays a role in mitotic recombination to remove non-homologous, 3′-ended overhangs from recombination intermediates. The Arabidopsis atrad1 mutant (uvh1), unlike rad1 mutants known from other eukaryotes, is hypersensitive to ionizing radiation. This last observation may indicate a more important role for the Rad1/Rad10 endonuclease in recombination in plants. This is the first direct demonstration of the involvement of AtRad1p in homologous recombination in plants.