The conversion from fast-growing tachyzoites to slow-growing bradyzoites is the key factor in establishing the chronic infection and long-term persistence of Toxoplasma gondii. Environmental stressors, such as amino acid starvation and alkaline medium, can trigger the transformation of tachyzoites into bradyzoites. Under such stress conditions, ribosomes slow down, potentially leading to stalling, and ribosomal collisions typically activate ribosome-associated quality control (RQC) pathways. In this study, we investigated the role of T. gondii ribosome quality control complex subunit 2 (TgRqc2), which contains both NFACT and coiled-coil domains, in the parasite's survival and stage conversion. NFACT represents the "domain" found in the central players involved in RQC, human NEMF and its orthologs FbpA (known as RqcH), Caliban, and Tae2 (known as Rqc2). Phylogenetic analyses revealed that TgRqc2 formed a distinct clade with its orthologs in apicomplexan parasites. The deletion of TgRqc2 impaired T. gondii's invasion and replication. The Rqc2-knockout strain showed defects in plaque formation and bradyzoite development. Our findings demonstrate that TgRqc2 is essential for T. gondii's lytic cycle and the conversion of tachyzoites into bradyzoites. RNA-seq analysis further showed that the depletion of TgRqc2 significantly disrupted global transcriptional activity. However, the detailed molecular mechanisms involved remain to be elucidated. In conclusion, our results proved valuable insights that may aid in the development of therapeutic strategies to prevent chronic infection.

Keywords: NFACT; TgRqc2; Toxoplasma gondii; bradyzoite; tachyzoite.