In an effort to determine the precise events taking place
during telomere replication in yeast, we discovered that mutations in
either gene encoding the yeast homologues of the Ku-complex lead to an
altered terminal DNA structure. In cells lacking Ku, telomeres have an
overhang of 50-90 bases of the 3'-end even in cells outside S-phase. In
addition, the transcriptional repression of genes located near
chromosome ends, but not of genes located at the silent mating type
loci, is abrogated in such strains. Finally, using an epitope-tagged
version of Yku80p and an in vivo crosslinking protocol, DNA
fragments containing telomeric repeats could be immunoprecipitated from
cell free extracts. These results show that yeast Ku plays a direct role
in maintaining a normal DNA end-structure on yeast chromosomes,
conceivably by functioning as a terminus-binding factor. Previous work
had established that Ku is an important factor for RAD52-independent
non-homologous end-joining of ds DNA breaks. However, telomere to
telomere end-joining reactions would lead to dicentric chromosomes and
genomic instability. We therefore suggest that yeast Ku is involved in
at least two distinct mechanisms, depending on the location of its
binding. We speculate that interactions of Ku with telomere associated
proteins or with proteins involved in NHEJ may play a crucial role in
distinguishing chromosome ends, where end-to-end fusions are not
desirable, from ds DNA breaks within a chromosome.
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