Table 1.
DSBR pathways and their possible contribution to antigenic variation in T. brucei.
| Pathway | Sub-pathway | Features | Proposed role in VSG switching | Ref. |
|---|---|---|---|---|
| Homologous recombination (HR) | Gene conversion (GC) | Copying and replacement of a segment of DNA using flanking homologies | Any VSG (fragment) with sufficient homology could be copied into the active ES by this typically RAD51-dependent mechanisma | [29] |
| Single-strand annealing (SSA) | Deletion of a segment of DNA using flanking homologies | None | ||
| Break-induced replication (BIR) | Copying and replacement of a segment of DNA to the chromosome end using a single region of homology | The subset of telomeric VSGs could be copied into the active ES by this typically RAD51-dependent mechanisma | [33] | |
| End-joining (EJ) | Non-homologous EJ (NHEJ) | Re-ligation of broken strands typically with small deletions | None — not seen in T. brucei | |
| Microhomology-mediated EJ (MMEJ, aka micro-SSA) | Deletion of a segment of DNA using flanking microhomologies of 5–20 bp. Gene conversion (see above) can be mediated by one-sided MMEJ | MMEJ-based equivalents of (one-sided) GC and BIR would be predicted to be RAD51-dependentand independent respectivelya (see above) | [32] | |
a
Recombination-based VSG switching operates via RAD51-dependent and independent mechanisms.