YEAST KU AS A REGULATOR OF CHROMOSOMAL DNA END STRUCTURE

During telomere replication in yeast, chromosome ends acquire an S-pha se-specific overhang of the guanosine-rich strand. Here it is shown th at in cells lacking Ku, a heterodimeric protein involved in nonhomolog ous DNA end joining, these overhangs are present throughout the cell c ycle. In vivo cross-linking experiments demonstrated that Ku is bound to telomeric DNA, These results show that Ku plays a direct role in es tablishing a normal DNA end structure on yeast chromosomes, conceivabl y by functioning as a terminus-binding factor. Because Ku-mediated DNA end joining involving telomeres would result in chromosome instabilit y, our data also suggest that Ku;has a distinct function when bound to telomeres.