THE SACCHAROMYCES-CEREVISIAE KU AUTOANTIGEN HOMOLOG AFFECTS RADIOSENSITIVITY ONLY IN THE ABSENCE OF HOMOLOGOUS RECOMBINATION

In mammalian cells, all subunits of the DNA-dependent protein kinase ( DNA-PK) have been implicated in the repair of DNA double-strand breaks and in V(D)J recombination. In the yeast Saccharomyces cerevisiae, we have examined the phenotype conferred by a deletion of HDF1, the puta tive homologue of the 70-kD subunit of the DNA-end binding Ku complex of DNA-PK. The yeast gene does not play a role in radiation-induced ce ll cycle checkpoint arrest in G(1) and G(2) or in hydroxyurea-induced checkpoint arrest in S. In cells competent for homologous recombinatio n, we could not detect any sensitivity to ionizing radiation or to met hyl methanesulfonate (MMS) conferred by a hdf1 deletion and indeed, th e repair of DNA double-strand breaks was not impaired. However, if hom ologous recombination was disabled (rad52 mutant background), inactiva tion of HDF1 results in additional sensitization toward ionizing radia tion and MMS. These results give further support to the notion that, i n contrast to higher eukaryotic cells, homologous recombination is the favored path was of double-strand break repair in yeast whereas other competing mechanisms such as the suggested pathway of DNA-PK-dependen t direct break rejoining are only of minor importance.