T-loops at trypanosome telomeres

Mammalian telomeres form large duplex loops (t-loops) that may sequester ch romosome ends by invasion of the 3' TTAGGG overhang into the duplex TTAGGG repeat array. Here we document t-loops in Trypanosoma brucei, a kinetoplast id protozoan with abundant telomeres due to the presence of many minichromo somes. These telomeres contained 10-20 kb duplex TTAGGG repeats and a 3' TT AGGG overhang. Electron microscopy of psoralen/UV crosslinked DNA revealed t-loops in enriched telomeric restriction fragments and at the ends of isol ated minichromosomes. In mammals, t-loops are large (up to 25 kb), often co mprising most of the telomere, Despite similar telomere lengths, trypanosom e t-loops were much smaller (similar to1 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 bo th telomeres and several cross-linked minichromosomes had t-loops at both e nds. 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.