Trypanosoma brucei has short telomere G-overhangs

Telomeres are specialized nucleoprotein complexes at the end of linear chromosomes. They are essential for chromosome stability. In most eukaryotes, telomere DNA consists of simple repetitive TG-rich sequences, and there is a single-stranded 3’ G-rich overhang at the very end of the telomere. This telomere G-overhang structure is essential for telomere maintenance: in the presence of telomerase, G-overhang serves as a primer; in the absence of telomerase, G-overhang can mediate efficient break-induced-repair (BIR) of chromosome ends, an alternative mechanism for telomere maintenance and an important pathway for telomere recombination. Telomere recombination is particularly important for Trypanosoma brucei, a protozoan parasite and the causative agent of Human African Trypanosomiasis. T. brucei evades the host’s immune attack by regularly changing its variant surface glycoprotein (VSG), and homologous recombination is an important mechanism for VSG switching. It is possible that abnormal telomere recombination may affect VSG switching efficiency. However, the telomere terminal structure in T. brucei is poorly understood, and so far native in-gel hybridization is the only method to detect telomere G-overhang signal in this organism. To further analyze the telomere G-overhang structure in T. brucei, we adopted a ligation-mediated primer extension assay. This allows us to determine both the length and the terminal nucleotide of the telomere G-overhang, and we observed that the majority of T. brucei telomeres have 10–20 nt long G-overhangs that end in 5’ TTAGGG 3’, but that a small number of telomeres have 40–60 nt long G-overhangs that end in 5’ GGGTTA 3’. In addition, T. brucei cells appear to have longer telomere G-overhangs at the bloodstream form than at the procyclic form stage.