RAD51

In Toxoplasma gondii (strain RH), RAD51 is the key recombinase of the homologous recombination repair (HRR) pathway, expressed in replicating tachyzoites where it forms DNA damage foci to resolve DNA double-strand breaks that arise during replication (PMID:40784138). Its activity is required for normal cell cycle progression: conditional AID-mediated knockdown disrupts tachyzoite replication synchronization (PMID:40784138), and pharmacological inhibition with B02 (IC50 4.8 µM) causes S-phase arrest (PMID:40784138). Beyond genome maintenance, RAD51 loss couples replication stress to developmental fate, since B02 inhibition induces tachyzoite-to-bradyzoite differentiation with formation of small cyst-like structures — indicating that RAD51-dependent genome maintenance suppresses stress-induced stage conversion (PMID:40784138). No further mechanistic detail on RAD51 partners, substrates, or the molecular basis of foci assembly has been characterized in the available corpus.

1. 2025 Medium

   Whether T. gondii possesses a functional HRR recombinase acting during replication was unknown; endogenous tagging showed RAD51 is expressed in replicating tachyzoites and assembles into DNA damage foci, establishing it as the key recombinase resolving replication-associated double-strand breaks.

   Evidence Endogenous HA tagging via AID system with immunofluorescence detection of damage foci in T. gondii RH tachyzoites

   PMID:40784138

   Open questions at the time

   • No interacting partners, presynaptic filament components, or substrates identified
   • Molecular trigger and composition of the damage foci not defined
   • No structural model of the recombinase or its filament
2. 2025 Medium

   It was unclear whether HRR activity is required for parasite proliferation; conditional depletion showed RAD51 loss disrupts replication synchronization, establishing that RAD51-mediated HRR is needed for normal tachyzoite cell cycle progression.

   Evidence AID-mediated conditional knockdown with cell cycle analysis in T. gondii RH

   PMID:40784138

   Open questions at the time

   • The specific cell cycle checkpoint engaged by RAD51 loss is not mapped
   • Whether the replication defect reflects unresolved DSBs versus a signaling role is not distinguished
3. 2025 Medium

   Whether replication stress could drive developmental conversion was unknown; pharmacological RAD51 inhibition caused S-phase arrest and induced tachyzoite-to-bradyzoite differentiation, linking genome maintenance failure to stress-induced stage conversion.

   Evidence Small-molecule inhibition with B02 (IC50 4.8 µM); growth, cell cycle, and stage conversion assays in T. gondii RH

   PMID:40784138

   Open questions at the time

   • B02 off-target effects not excluded by genetic epistasis for the differentiation phenotype
   • Signaling pathway connecting S-phase arrest to bradyzoite conversion not defined
   • Whether cyst-like structures represent bona fide mature bradyzoites in vivo is untested

• The molecular interactors, upstream loaders, and downstream effectors that mediate RAD51 filament assembly and the link between replication stress and stage conversion remain undefined.
• No partner proteins or mediator/loader factors identified
• Mechanism coupling S-phase arrest to bradyzoite differentiation unknown
• No cross-strain comparison beyond RH