YEAST DNA-LIGASE-IV MEDIATES NONHOMOLOGOUS DNA END JOINING

The discovery of homologues from the yeast Saccharomyces cerevisiae of the human Ku DNA-end-binding proteins (HDF1 and KU80) has established that this organism is capable of non-homologous double-strand end joi ning (NHET)(1-5), a form of DNA double-strand break repair (DSBR) acti ve in mammalian V(D)J recombination(6-8). Identification of the DNA li gase that mediates NHEJ in yeast will help elucidate the function of t he four mammalian DNA ligases in DSBR, V(D)J recombination and other r eactions(9,10). Here we show that S. cerevisiae has two typical DNA li gases, the known DNA ligase I homologue CDC9 (refs 11-14) and the prev iously unknown DNA ligase IV homologue DNL4. dnl4 mutants are deficien t in precise and end-processed NHEJ. DNL4 and HDF1 are epistatic in th is regard, with the mutation of each having equivalent effects. dn14 m utants are complemented by overexpression of Dn14 but not of Cdc9, and deficiency of Dn14 alone does not impair either cell growth or the Cd c9-mediated responses to ionizing and ultraviolet radiation. Thus, S. cerevisiae has two distinct and separate Ligation pathways.