Sequencing of the expressed VSGs in T. brucei cells where monoallelic VSG expression has been disrupted.. VSG seq T. brucei

In the mammalian host, Trypanosoma brucei is coated in a single variant surface glycoprotein (VSG) species and stochastic switching allows the parasite to escape detection by the host immune system. The major pathway for VSG switching is via genetic recombination, however little is known about how the parasite prevents multiple VSG switching events occurring simultaneously, which would interfere with monoallelic expression. We show that disruption of monoallelic VSG expression can be achieved through stable ectopic expression of a second telomeric VSG and by triggering switching through the induction of a DNA double strand break (DSB), strict monoallelic expression is restored. We found that in cells expressing two VSG genes the DNA sequence of the ectopic copy is lost following a DSB, despite it being spatially separated in the genome. The loss of ectopic VSG is dependent on active transcription and does not disrupt the number or variety of templates used to repair a BES DSB. We propose that there are stringent mechanisms within the cell to reinforce monoallelic expression during antigenic variation.