African trypanosomes are mammalian pathogens that must regularly change their protein coat to survive in the host bloodstream. Chronic trypanosome infections are potentiated by their ability to access a deep genomic repertoire of Variant Surface Glycoprotein ( VSG ) genes and switch from the expression of one VSG to another. Switching VSG expression is largely based in DNA recombination events that result in chromosome translocations between an acceptor site, which houses the actively transcribed VSG , and a donor gene, drawn from an archive of more than 2,000 silent VSG s. One element implicated in these duplicative gene conversion events is a DNA repeat of approximately 70 bp that is found in long regions within each BES and short iterations proximal to VSG s within the silent archive. Early observations showing that 70-bp repeats can be recombination boundaries during VSG switching led to the prediction that VSG -proximal 70-bp repeats provide recombinatorial homology. Yet, this long held assumption had not been tested and no specific function for the conserved 70-bp repeats had been demonstrated. In the present study, the 70-bp repeats were genetically manipulated under conditions that induce gene conversion. In this manner, we demonstrated that 70-bp repeats promote access to archival VSG s. Synthetic repeat DNA sequences were then employed to identify the length, sequence, and directionality of repeat regions required for this activity. In addition, manipulation of the 70-bp repeats allowed us to observe a link between VSG switching and the cell cycle that had not been appreciated. Together these data provide definitive support for the long-standing hypothesis that 70-bp repeats provide recombinatorial homology during switching. Yet, the fact that silent archival VSGs are selected under these conditions suggests the 70-bp repeats also direct DNA pairing and recombination machinery away from the closest homologs (silent BESs) and toward the rest of the archive. Author Summary Chromosomal translocations can fuel genetic change or cause catastrophic genomic damage. African trypanosomes, exemplified by Trypanosoma brucei sub-species, are unicellular parasites that can chronically infect their human and livestock hosts by using a strategy of antigenic variation by which they repeatedly change their protein coats. Switching the surface coat requires the accurate selection and translocation of a single silent coat gene, from a large genomic archive, into an actively transcribed site. How the coat genes from within this deep archive are selected and activated was unproven. Here we show that a specific repetitive DNA sequence is required to access coat genes from diverse sites within the genome. The likely outcome of restricting this process of coat gene selection in natural infections would be a reduction in the chronic nature of African trypanosomiasis.
【초록키워드】 Genome, Infection, outcome, cell cycle, Surface glycoprotein, Protein, DNA, Region, pathogen, African, Recombination, Pathogens, surface, natural infection, genomic, expression, synthetic, Donor, Hypothesis, antigenic variation, switching, chronic, the cell, gene conversion, Trypanosoma brucei, Variant Surface Glycoprotein, Chromosomal translocations, Support, observation, African trypanosomiasis, chromosome, These data, species, sequence, bloodstream, homology, assumption, parasite, DNA sequence, recombination event, homolog, archive, genetic change, manipulation, translocation, trypanosomiasis, Trypanosoma, mammalian, infect, Host, Cell, event, catastrophic, observé, selected, tested, identify, conserved, addition, required, activated, condition, demonstrated, promote, induce, transcribed, reduction in, implicated, silent, proximal, Chromosomal translocation, iteration, Trypanosome, VSG, 【제목키워드】 DNA, Selection, surface, Repeat, Trypanosoma brucei, repertoire, Trypanosoma,