Arabidopsis DNA ligase IV is induced by gamma-irradiation and interacts with an Arabidopsis homologue of the double strand break repair protein XRCC4

Rejoining of single- and double-strand breaks (DSBs) introduced in DNA duri ng replication, recombination, and DNA damage is catalysed by DNA ligase en zymes. Eukaryotes possess multiple DNA ligase enzymes, each having distinct roles in cellular metabolism. Double-strand breaks in DNA, which can occur spontaneously in the cell or be induced experimentally by gamma-irradiatio n, represent one of the most serious threats to genomic integrity. Non-homo logous end joining (NHEJ) rather than homologous recombination is the major pathway for repair of DSBs in organisms with complex genomes, including hu mans and plants. DNA ligase IV in Saccharomyces cerevisiae and humans catal yses the final step in the NHEJ pathway of DSB repair. In this study we ide ntify an Arabidopsis thaliana homologue (AtLIG4) of human and S. cerevisiae DNA ligase IV which is shown to encode an ATP-dependent DNA ligase with a theoretical molecular mass of 138 kDa and 48% similarity in amino-acid sequ ence to the human DNA ligase IV. Yeast two-hybrid analysis demonstrated a s trong interaction between A. thaliana DNA ligase IV and the A. thaliana hom ologue of the human DNA ligase IV-binding protein XRCC4. This interaction i s shown to be mediated via the tandem BRCA C-terminal domains of A. thalian a DNA ligase IV protein. Expression of AtLIG4 is induced by gamma-irradiati on but not by UVB irradiation, consistent with an in vivo role for the A. t haliana DNA ligase IV in DSB repair.