Figure 2.

DNA Replication and Transcription Intersect to Drive Antigenic Variation in the Trypanosoma brucei VSG Expression Sites.

(A) Current understanding of DNA replication in the actively transcribed T. brucei VSG ES. A simplified VSG ES is shown, with key features indicated: RNA Pol I promoter, transcription direction (red arrow), ESAGs (blue boxes), 70 bp repeats (hatches), VSG (red arrow), and telomere (array of arrows). Two possibilities for the direction of ORC-derived DNA replication are shown (black arrows), which can result in codirectional or head on collisions with transcription. ORC binding in the ES has not been mapped, however, and so it remains possible that DNA replication is ORC-independent (not shown). (B) A model for VSG recombination during antigenic variation. Transcription is impeded by DNA replication, here shown as a head on collision and focused on the 70 bp repeats. Pausing of RNA Pol I leads to the formation of an RNA–DNA hybrid (R loop), in which RNase H1 and RNase H2 (yellow circle) hydrolyse the RNA to resolve the structure. ATR (green circle) may recognise the R loop. Together, these activities contribute to the generation of DNA breaks in the ES, which are repaired by gene conversion from a silent VSG (pink arrow, here shown in a subtelomeric array) into the ES, replacing the previously expressed VSG. Abbreviations: ATR, Ataxia telangiectasia and Rad3-related; ES, expression site; EASG, ES-associated gene; ORC, origin recognition complex; RNA Pol, RNA polymerase; VSG, variant surface glycoprotein.