TgAP2IX-5 is a key transcriptional regulator of the asexual cell cycle division in Toxoplasma gondii.

Apicomplexan parasites have evolved efficient and distinctive strategies for intracellular replication where the timing of emergence of the daughter cells (budding) is a decisive element. However, the molecular mechanisms that provide the proper timing of parasite budding remain unknown. Using Toxoplasma gondii as a model Apicomplexan, we identified a master regulator that controls the timing of the budding process. We show that an ApiAP2 transcription factor, TgAP2IX-5, controls cell cycle events downstream of centrosome duplication. TgAP2IX-5 binds to the promoter of hundreds of genes and controls the activation of the budding-specific cell cycle expression program. TgAP2IX-5 regulates the expression of specific transcription factors that are necessary for the completion of the budding cycle. Moreover, TgAP2IX-5 acts as a limiting factor that ensures that asexual proliferation continues by promoting the inhibition of the differentiation pathway. Therefore, TgAP2IX-5 is a master regulator that controls both cell cycle and developmental pathways.

Cooperative binding of ApiAP2 transcription factors is crucial for the expression of virulence genes in Toxoplasma gondii.

Toxoplasma gondii virulence depends on the expression of factors packed into specific organelles such as rhoptry and microneme. Although virulence factor expression is tightly regulated, the molecular mechanisms controlling their regulation remain poorly understood. ApiAP2 are a family of conserved transcription factors (TFs) that play an important role in regulating gene expression in apicomplexan parasites. TgAP2XI-5 is able to bind to transcriptionally active promoters of genes expressed during the S/M phase of the cell cycle, such as virulence genes (rhoptries and micronemes genes). We identified proteins interacting with TgAP2XI-5 including a cell cycle-regulated ApiAP2 TF, TgAP2X-5. Using an inducible knock-down strategy and RNA-seq, we demonstrated that the level of expression of number of virulence factors transcripts is affected by the disruption of TgAP2X-5 expression. While TgAP2X-5 disruption has mild effect on parasite invasion, it leads to the strain avirulence in mice. To better understand the molecular mechanisms at stake, we investigated the binding of TgAP2XI-5 at promoters in the TgAP2X-5 mutant strain in a genome-wide assay. We show that disruption of TgAP2X-5 expression leads to defects in TgAP2XI-5 binding to multiple rhoptry gene promoters. Taken together, these data suggest a cooperative contribution of two ApiAP2 TF in the regulation of virulence genes in T. gondii.