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Molecular and Cellular Biology, May 2004, p. 4571-4580, Vol. 24, No. 10
0270-7306/04/$08.00+0     DOI: 10.1128/MCB.24.10.4571-4580.2004
Copyright © 2004, American Society for Microbiology. All Rights Reserved.

Modification of Subtelomeric DNA

Department of Cell Biology, University of Texas Southwestern Medical Center, Dallas, Texas 75390-9039

Received 22 October 2003/ Returned for modification 3 December 2003/ Accepted 24 February 2004

There is a discrepancy in telomere length as measured by signal intensity of telomere restriction fragments on gels and fluorescence in situ hybridization analysis. This difference has been ascribed to the X-region, a segment of subtelomeric DNA that is resistant to being cut by restriction enzymes. To explore the nature of this region, we analyzed the digestibility of an artificial seeded telomere in HeLa cells as well as the Xp/Yp autosomal telomere in human BJ fibroblasts. We found that there is a substantial fraction of subtelomeric DNA containing restriction sites that is not digested with enzymes such as EcoRI, NlaIII, and SphI. Comparison of methylation-sensitive and -resistant enzymes excluded the possibility of the X-region being maintained by DNA methylation. We show that the X-region represents a variable domain whose size changes with telomere length, and neither non-TTAGGG sequences nor cytidine methylation can adequately explain the size of the X-region.