Organization of telomeres during the cell and life cycles of Trypanosoma brucei

The genome of Trypanosoma brucei contains about 120 chromosomes, which do n ot visibly condense during mitosis. We have analyzed the organization and s egregation of these chromosomes by in situ hybridization using fluorescent telomere probes. At the onset of mitosis. telomeres migrate from their nucl ear peripheral location and congregate into a central zone. This dense grou p of telomeres then splits into two entities that migrate to opposite nucle ar poles. Segregation continues until the double-sized nucleus divides and, before cytokinesis occurs, the,telomeres reorganize into the discrete foci observed at interphase. During migration, the telomeres are located at the free end of the mitotic spindle. Treatment with the microtubule polymeriza tion inhibitor rhizoxin prevents telomere clustering and chromosomal segreg ation. In the insect-specific procyclic form as well as in the non-dividing bloodstream stumpy form, telomeres tend to cluster close to the nuclear pe riphery at interphase. In contrast, in the proliferative bloodstream slende r form the telomeres preferentially locate in the central zone of the nucle us. Thus, telomeres are closer to the nuclear periphery during those life c ycle stages where the telomeric expression sites For the variant surface gl ycoprotein are all inactive, suggesting that transcriptional inactivation o f these sites is related to their subnuclear localization.