Toxoplasma transcription factor TgAP2XI-5 regulates the expression of genes involved in parasite virulence and host invasion.

Gene regulation in apicomplexan parasites, a phylum containing important protozoan parasites such as Plasmodium and Toxoplasma, is poorly understood. The life cycle of Toxoplasma gondii is complex, with multiple proliferation and differentiation steps, of which tachyzoite proliferation is the most relevant to pathogenesis in humans and animals. Tachyzoites express invasion and virulence factors that are crucial for their survival and manipulation of host cell functions. The expression of those factors is tightly controlled during the tachyzoite cell cycle to permit their correct packaging in newly formed apical secretory organelles named micronemes and rhoptries in the daughter cells. However, little is known about the factors that control the expression of genes encoding the virulence factors present in these parasite-specific secretory organelles. We report that the plant-like nuclear factor TgAP2XI-5 targets more than 300 gene promoters and actively controls the transcription of these genes. Most of these target genes, including those that are essential for parasite virulence, showed a peak of expression in the S and M phases of the cell cycle. Furthermore, we identified the cis-regulatory element recognized by TgAP2XI-5 and demonstrated its ability to actively drive gene transcription. Our results demonstrated that TgAP2XI-5 is a novel DNA sequence-specific transcription factor associated with promoter activation. TgAP2XI-5 may regulate gene transcription of crucial virulence factors in T. gondii.