Abstract
Subtelomeric regions are often under-represented in genome sequences of eukaryotes. One of the best known examples of the use of telomere proximity for adaptive purposes are the bloodstream expression sites (BESs) of the African trypanosome Trypanosoma brucei. To enhance our understanding of BES structure and function in host adaptation and immune evasion, the BES repertoire from the Lister 427 strain of T. brucei were independently tagged and sequenced. BESs are polymorphic in size and structure but reveal a surprisingly conserved architecture in the context of extensive recombination. Very small BESs do exist and many functioning BESs do not contain the full complement of expression site associated genes (ESAGs). The consequences of duplicated or missing ESAGs, including ESAG9, a newly named ESAG12, and additional variant surface glycoprotein genes (VSGs) were evaluated by functional assays after BESs were tagged with a drug-resistance gene. Phylogenetic analysis of constituent ESAG families suggests that BESs are sequence mosaics and that extensive recombination has shaped the evolution of the BES repertoire. This work opens important perspectives in understanding the molecular mechanisms of antigenic variation, a widely used strategy for immune evasion in pathogens, and telomere biology.
Highlights
Subtelomeres are dynamic and fast-evolving regions of eukaryotic genomes owing to their remarkable plasticity [1,2,3]
Have we found all of the bloodstream expression sites (BESs) in T. brucei 427? Assuming that the promoter sequence is conserved in all BESs, we estimated the chance of missing any BESs in the entire library by simulating samples of 182 randomly sampled clones from populations with different numbers of underlying BESs
No BESs were tagged that were absent from the collection of transformation-associated recombination (TAR) clones and we are confident that the library contains the entire T. brucei Lister 427 BES repertoire
Summary
Subtelomeres are dynamic and fast-evolving regions of eukaryotic genomes owing to their remarkable plasticity [1,2,3]. Recombination between internal repeats and chromosomal arms results in the accumulation of species-specific sequences, commonly mediating adaptation to the environment. Despite their extreme genetic diversity, common aspects of structure and function are shared across telomeres from a diverse range of organisms [4]. The ability to encode contingency functions within their subtelomeric regions has been exploited by the African Trypanosome, subspecies of which cause the debilitating disease Human African Trypanosomiasis and the related disease ‘nagana’ in livestock [reviewed in [2]. Trypanosoma brucei evades the mammalian host’s immune response by periodically changing its variant surface glycoprotein (VSG) coat (reviewed in: [5]), a dense monolayer of 56106 identical VSG dimers. To change the VSG coat, transcription is switched to an alternative VSG by recombination of a new VSG into the active BES or by transcriptional silencing of one BES with concomitant activation of a second
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