Exonuclease activity is required for sequence addition and Cdc13p loading at a de novo telomere

Curr Biol. 2001 Sep 4;11(17):1336-40. doi: 10.1016/s0960-9822(01)00400-6.


The Saccharomyces cerevisiae Mre11p/Rad50p/Xrs2p (MRX) complex is evolutionarily conserved and functions in DNA repair and at telomeres [1-3]. In vivo, MRX is required for a 5' --> 3' exonuclease activity that mediates DNA recombination at double-strand breaks (DSBs). Paradoxically, abolition of this exonuclease activity in MRX mutants results in shortened telomeric DNA tracts. To further explore the role of MRX at telomeres, we analyzed MRX mutants in a de novo telomere addition assay in yeast cells [4]. We found that the MRX genes were absolutely required for telomerase-mediated addition in this assay. Furthermore, we found that Cdc13p, a single-stranded telomeric DNA binding protein essential for telomere DNA synthesis and protection [5], was unable to bind to the de novo telomeric DNA substrate in cells lacking Rad50p. Based on the results from this model system, we propose that the MRX complex helps to prepare telomeric DNA for the loading of Cdc13p, which then protects the chromosome from further degradation and recruits telomerase and other DNA replication components to synthesize telomeric DNA.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Cyclin B / metabolism*
  • DNA Damage
  • DNA-Binding Proteins*
  • Endodeoxyribonucleases / genetics
  • Endodeoxyribonucleases / metabolism*
  • Endodeoxyribonucleases / physiology
  • Exodeoxyribonuclease V
  • Exodeoxyribonucleases / genetics
  • Exodeoxyribonucleases / metabolism*
  • Exodeoxyribonucleases / physiology
  • Fungal Proteins / genetics
  • Fungal Proteins / metabolism*
  • Fungal Proteins / physiology
  • Saccharomyces cerevisiae / genetics
  • Saccharomyces cerevisiae / metabolism
  • Saccharomyces cerevisiae Proteins*
  • Telomere / metabolism*
  • Telomere / physiology


  • Cyclin B
  • DNA-Binding Proteins
  • Fungal Proteins
  • RAD50 protein, S cerevisiae
  • Saccharomyces cerevisiae Proteins
  • XRS2 protein, S cerevisiae
  • Endodeoxyribonucleases
  • Exodeoxyribonucleases
  • MRE11 protein, S cerevisiae
  • Exodeoxyribonuclease V