MUTATION OF YEAST KU GENES DISRUPTS THE SUBNUCLEAR ORGANIZATION OF TELOMERES

The mammalian Ku70 and Ku86 proteins form a heterodimer that binds to the ends of double-stranded DNA in vitro and is required for repair of radiation-induced strand breaks and V(D)J recombination [1,2], Deleti on of the Saccharomyces cerevisiae genes HDF1 and HDF2 - encoding yKu7 0p and yKu80p, respectively enhances radiation sensitivity in a rad52 background [3,4]. In addition to repair defects, the length of the TG- rich repeat on yeast telomere ends shortens dramatically [5,6], We hav e shown previously that in yeast interphase nuclei, telomeres are clus tered in a limited number of foci near the nuclear periphery [7], but the elements that mediate this localization remained unknown. We repor t here that deletion of the genes encoding yKu70p or its partner yKu80 p altered the positioning of telomeric DNA in the yeast nucleus, These are the first mutants shown to affect the subnuclear localization of telomeres, Strains deficient for either yKu70p or yKu80p lost telomeri c silencing, although they maintained repression at the silent mating- type loci. In addition, the telomere-associated silencing factors Sir3 p and Sir4p and the TG repeat-binding protein Rap1p lost their punctat e pattern of staining and became dispersed throughout the nucleoplasm. Our results implicate the yeast Ku proteins directly in aspects of te lomere organization, which in turn affects the repression of telomere proximal genes. (C) Current Biology Ltd ISSN 0960-9822.