<scp>AP2XI</scp> ‐3 is Critical for the Cell Cycle of <i>Toxoplasma</i> to Progress Through the <scp>G1</scp> Phase

Toxoplasmosis caused by the Apicomplexan parasite Toxoplasma gondii is a significant health threat to immunocompromised individuals and newborns. This parasite has a complex life cycle that is well controlled to achieve optimal transmission and pathogenesis. Acute phase of the disease is caused by rapid proliferation of tachyzoites, which has a highly coordinated and tightly regulated cell cycle to allow parasite propagation. Tachyzoite cell cycle has five partially overlapping phases (G1, S, G2, M and C) with distinct gene expression patterns and cellular activities, yet the underlying regulatory mechanisms are not well understood. In this study, we show that the AP2 family transcription factor AP2XI-3 has important roles in regulating the cell cycle to progress through the G1 phase. Depletion of AP2XI-3 resulted in cell cycle retention at G1, leading to growth inhibition of tachyzoites. RNA-Seq and CUT&Tag analyses revealed that AP2XI-3 regulates the transcription of genes involved in nucleic acid metabolism, RNA biogenesis and processing, which are consistent with the cellular activities of G1 phase in preparing biomass for cell cycle progression. Moreover, AP2XI-3 binds to the conserved DNA motif, TRP-2, a sequence widely distributed in the promoters of genes that exhibit peak expression during the G1 phase. Together, these findings underscore the essential role of AP2XI-3 in the tight regulation of the G1 phase, highlighting its potential as a therapeutic target for drug development.