IDENTIFICATION OF A SACCHAROMYCES-CEREVISIAE KU80 HOMOLOG - ROLES IN DNA DOUBLE-STRAND BREAK REJOINING AND IN TELOMERIC MAINTENANCE

Ku is a heterodimer of polypeptides of approximately 70 and 80 kDa (Ku 70 and Ku80, respectively) that binds to DNA ends, Mammalian cells lac king Ku are defective in DNA double-strand break (DSB) repair and in s ite-specific V(D)J recombination, Here, we describe the identification and characterisation of YKU80,the gene for the Saccharomyces cerevisi ae Ku80 homologue, Significantly, we find that YKU80 disruption enhanc es the radiosensitivity of rad52 mutant strains, suggesting that YKU80 functions in a DNA DSB repair pathway that does not rely on homologou s recombination, Indeed, through using an in vivo plasmid rejoining as say, we find that YKU80 plays an essential role in illegitimate recomb ination events that result in the accurate repair of restriction enzym e generated DSBs, Interestingly, in the absence of YKU80 function, res idual repair operates through an error-prone pathway that results in r ecombination between short direct repeat elements. This resembles clos ely a predominant DSB repair pathway in vertebrates, Together, our dat a suggest that multiple, evolutionarily conserved mechanisms for DSB r epair exist in eukaryotes, Furthermore, they imply that Ku binds to DS Bs in vivo and promotes repair both by enhancing accurate DNA end join ing and by suppressing alternative error-prone repair pathways, Finall y, we report that yku80 mutant yeasts display dramatic telomeric short ening, suggesting that, in addition to recognising DNA damage, Ku also binds to naturally occurring chromosomal ends, These findings raise t he possibility that Ku protects chromosomal termini from ii nucleolyti c attack and functions as part of a telomeric length sensing system.