A Novel Nuclear Protein Complex Controlling the Expression of Developmentally Regulated Genes in <i>Toxoplasma Gondii</i>

Toxoplasma gondii is a ubiquitous protozoan parasite with a complex life cycle containing multiple developmental stages. The parasites have distinct gene expression patterns at different stages to enable stage specific life activities, but the underlying regulatory mechanisms are largely unknown. In this study, a nuclear complex is identified that controls the expression of developmentally regulated genes. This complex consists of the AP2 family transcription factor AP2XII-5, the epigenetic factors MORC and HDAC3, as well as a novel AP2XII-5 interacting protein 1 (AIP1) that stabilizes this complex. At the tachyzoite stage when the parasites proliferate rapidly by asexual endodyogeny, AP2XII-5 binds to the promoter regions of developmentally activated genes and recruits MORC and HDAC3 to suppress their expression. When sexual commitment and merozoite development are triggered, the abundance of AP2XII-5 decreases and its suppression on target genes is relieved. In contrast to MORC and HDAC3, which regulate Toxoplasma development but are also essential for tachyzoite growth, AP2XII-5 and AIP1 are dispensable for tachyzoite proliferation in vitro. These data suggest that while MORC and HDAC3 have broad regulatory activities, forming a complex with AP2XII-5 and AIP1 enables them to specifically regulate gene expression during development.