t-loops at trypanosome telomeres

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 (∼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.