Chromosomes Ends and Telomere Biology of Trypanosomatids

In this chapter, we focus on the structure and function of telomeres and subtelomeres of human protozoan parasites T. cruzi, T. rangeli and Leishmania spp.. Beyond their role in maintaining the integrity of chromosomes, telomeres and subtelomeres are involved in the survival mechanisms of these single-celled parasites. The telomeric repeat (5'-TTAGGG-3')n is conserved among trypanosomatid species, but adjacent subtelomers vary between species and chromosomes within the same cell. The chromosome ends of T. rangeli, for example, exhibit a simple organization with short subtelomeres whereas T. cruzi subtelomeres are a complex mosaic of genomic fragments including gene/pseudogenes corresponding to large gene families of surface proteins and retrotransposons. Differences in the copy number and organization of these genes determine the variation in the size of subtelomeres on each T. cruzi chromosome. Leishmania subtelomeres, in contrast, lack genes encoding surface antigens; instead they carry conserved repeat sequences referred to as telomereassociated sequences. T. cruzi and T. rangeli chromosomes share a high level of synteny which is lost in the subtelomeric regions. It has been suggested that T. cruzi subtelomeres can serve as recombination hotspots and thus promoting the increase of the repertoire of surface antigens. Many pieces of evidence indicate that telomere maintenance in Kinetoplastids occurs primarily by a telomerase-mediated elongation. The catalytic subunit of telomerase (TERT) is present in all sequenced trypanosomatid species, whereas the RNA component containing a template for telomere repeat extension has recently been identified in T. brucei and Leishmania. Further studies are needed to understanding the regulation of telomere homeostasis and the biology of subtelomeres of trypanosomatids.

Keywords: Base modification, Chromosomes ends, Contingency genes, Retrotransposons, Subtelomere, Surface protein genes, Telomerase, Telomere generation, Telomere junction, Telomere repeats.