t-loops at trypanosome telomeres

EMBO J. 2001 Feb 1;20(3):579-88. doi: 10.1093/emboj/20.3.579.


Mammalian telomeres form large duplex loops (t-loops) that may sequester chromosome ends by invasion of the 3' TTAGGG overhang into the duplex TTAGGG repeat array. Here we document t-loops in Trypanosoma brucei, a kinetoplastid protozoan with abundant telomeres due to the presence of many minichromosomes. These telomeres contained 10-20 kb duplex TTAGGG repeats and a 3' TTAGGG overhang. Electron microscopy of psoralen/UV cross-linked DNA revealed t-loops in enriched telomeric restriction fragments and at the ends of isolated minichromosomes. In mammals, t-loops are large (up to 25 kb), often comprising most of the telomere. Despite similar telomere lengths, trypanosome t-loops were much smaller (approximately 1 kb), indicating that t-loop sizes are regulated. Coating of non-cross-linked minichromosomes with Escherichia coli single-strand binding protein (SSB) often revealed 3' overhangs at both telomeres and several cross-linked minichromosomes had t-loops at both ends. These results suggest that t-loops and their prerequisite 3' tails can be formed on the products of both leading and lagging strand synthesis. We conclude that t-loops are a conserved feature of eukaryotic telomeres.

Publication types

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

MeSH terms

  • Animals
  • Base Sequence
  • DNA, Protozoan / chemistry
  • DNA, Protozoan / genetics
  • DNA, Protozoan / ultrastructure
  • Microscopy, Electron
  • Oligonucleotide Probes / genetics
  • Tandem Repeat Sequences
  • Telomere / chemistry*
  • Telomere / genetics*
  • Telomere / ultrastructure
  • Trypanosoma brucei brucei / chemistry*
  • Trypanosoma brucei brucei / genetics*


  • DNA, Protozoan
  • Oligonucleotide Probes