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Mol Cell. 2011 Apr 22;42(2):224-36. doi: 10.1016/j.molcel.2011.03.015.

Mammalian 5' C-rich telomeric overhangs are a mark of recombination-dependent telomere maintenance.

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Molecular and Cellular Biology Laboratory, The Salk Institute for Biological Studies, 10010 North Torrey Pines Road, La Jolla, CA 92037, USA.


Recent evidence for 5'-cytosine (C)-rich overhangs at the telomeres of the nematode Caenorhabditis elegans provided the impetus to re-examine the end structure of mammalian telomeres. Two-dimensional (2D) gel electrophoresis, single telomere-length analysis (STELA), and strand-specific exonuclease assays revealed the presence of a 5'-C-rich overhang at the telomeres of human and mouse chromosomes. C-overhangs were prominent in G1/S arrested as well as terminally differentiated cells, indicating that they did not represent replication intermediates. C-rich overhangs were far more prevalent in tumor cells engaged in the alternative lengthening of telomeres (ALT) pathway of telomere maintenance, which relies on the homologous recombination (HR) machinery. Transient siRNA-based depletion of the HR-specific proteins RAD51, RAD52, and XRCC3 resulted in changes in C-overhang levels, implicating the involvement of 5'-C-overhangs in the HR-dependent pathway of telomere maintenance.

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