Divergent SAGA Complexes Shape the Toxoplasma Transcriptome across Host-Responsive and Core Gene Networks

However, how this parasite organizes and deploys its acetyltransferase machinery has remained unclear. Here, we show that T. gondii rewired this process using a plant-like system built around the acetyltransferase TgGCN5b, which differs from the typical SAGA complex found in other eukaryotes.

Proteomic and structural analyses reveal a modular assembly that integrates multiple acetyltransferase (GNAT) enzymes and chromatin-reader proteins carrying PHD and PZP domains and Apetala-related transcription factors, an organization unique to apicomplexan parasites. TgGCN5b catalyzes a selective tri-site acetylation pattern on histone H3 at lysines 9, 14, and 18 that maintains open chromatin and sustains transcription of core metabolic, invasion, and virulence genes.