Coordinated Progression Through Two Subtranscriptomes Underlies the Tachyzoite Cycle of Toxoplasma gondii

We describe the cell cycle transcriptome of the Toxoplasma gondii that has emerged as a major genetic model for the study of Apicomplexa parasites. Two distinct transcriptional waves accompany the relatively simple binary replication of tachyzoite stage (endodyogeny) functionally separating conserved gene expression in the eukaryotic G1 phase from the lineage-specific expression that predominates in the parasite S phase and mitotic periods. This division of transcriptional focus closely mirrors the intimate relationship that has evolved between mitosis and building of the daughter parasites and invasion organelles. Promoter mechanisms appear to orchestrate the induction of gene expression in dividing tachyzoites with up to two dozen cell cycle AP2 factors likely acting within a transcriptional regulatory network to coordinate the parasite cell cycle transcriptome. To characterize the cell cycle transcriptome of Toxoplasma tachyzoites, we expanded a thymidine-synchrony model to isolate sufficient RNA for microarray expression studies. The ToxoGeneChip microarray (http://ancillary.toxodb.org/docs/Array-Tutorial.html) was used to measure mRNA expression in 13 duplicate samples (R0 to R12) covering 12 hours post-synchronization and nearly two tachyzoite replication cycles.