Figure 1.

Model for homologous-recombination-mediated double-strand break repair and VSG switching. Center, a double-strand break (scissors) within the 70 bp repeats (yellow bar) at the active expression site initiates a search for a homologous repair template. Telomeric (T₂AG₃) repeats are indicated by the black arrowheads. Top, a silent telomeric VSG (up to 250 are available) can be copied into the active expression site via a gene-conversion cassette mechanism or by break-induced replication extending to the end of the chromosome. The longer stretches of 70 bp repeats found at these loci may increase the likelihood that these tracts are recognized and invaded by the broken strand at the active expression site. Bottom, replacement by a silent array VSG must use a cassette mechanism. The synthesis-dependent strand-annealing model [25] favors the type of recombination illustrated rather than crossover between silent VSG arrays and expression sites.