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

Abstract

BackgroundApicomplexan parasites replicate by varied and unusual processes where the typically eukaryotic expansion of cellular components and chromosome cycle are coordinated with the biosynthesis of parasite-specific structures essential for transmission.Methodology/Principal FindingsHere we describe the global cell cycle transcriptome of the tachyzoite stage of Toxoplasma gondii. In dividing tachyzoites, more than a third of the mRNAs exhibit significant cyclical profiles whose timing correlates with biosynthetic events that unfold during daughter parasite formation. These 2,833 mRNAs have a bimodal organization with peak expression occurring in one of two transcriptional waves that are bounded by the transition into S phase and cell cycle exit following cytokinesis. The G1-subtranscriptome is enriched for genes required for basal biosynthetic and metabolic functions, similar to most eukaryotes, while the S/M-subtranscriptome is characterized by the uniquely apicomplexan requirements of parasite maturation, development of specialized organelles, and egress of infectious daughter cells. Two dozen AP2 transcription factors form a series through the tachyzoite cycle with successive sharp peaks of protein expression in the same timeframes as their mRNA patterns, indicating that the mechanisms responsible for the timing of protein delivery might be mediated by AP2 domains with different promoter recognition specificities.Conclusion/SignificanceUnderlying each of the major events in apicomplexan cell cycles, and many more subordinate actions, are dynamic changes in parasite gene expression. The mechanisms responsible for cyclical gene expression timing are likely crucial to the efficiency of parasite replication and may provide new avenues for interfering with parasite growth.

Toxoplasma Transcription Factor TgAP2XI-5 Regulates the Expression of Genes Involved in Parasite Virulence and Host Invasion

“…Calculation of Normalized Cell Cycle Transcript ProfilesTranscript microarray data generated from Behnke et al (2) were downloaded from ToxoDB, the toxoplasma genomics resource. Robust multiarray average (RMA) 3 Toxoplasma values represent the level of expression for each gene.…”

“…These changes are accompanied by the regulation of different gene populations. Transcriptome studies reveal that 18% of genes present in the T. gondii genome are co-regulated at specific stages of the life cycle (1), and more than 2,500 genes show regulated expression during the tachyzoite cell cycle (2). The distribution of these co-regulated genes throughout the T. gondii genome strongly implies that cis-acting elements and trans-acting factors regulate the transcription of individual genes.…”

“…Intriguingly, the 68 putative AP2 transcription factors currently annotated in the T. gondii genome represent a significant expansion when compared with the 26 conserved AP2 proteins present in Plasmodium genomes. Importantly, like many tachyzoite genes, the steady-state transcript levels of 24 T. gondii AP2 genes appear to be cell cycle-dependent (2), implying that other members of the ApiAP2 (apicomplexan AP2) family may also participate in regulating the expression of stage-specific genes in Apicomplexa. Recently, we and others have demonstrated the involvement of a T. gondii AP2 transcription factor in cyst formation, indicating that ApiAP2 factors may play an important role in T. gondii gene regulation (7,8).…”

Molecular target validation, antimicrobial delivery, and potential treatment of Toxoplasma gondii infections

“…This family of ApiAP2 transcription factors regulates cell cycle, replication, and maintenance of the tachyzoite life-cycle stage. These transcription factors also regulate differentiation, and switch to and maintenance of the bradyzoite life-cycle stage in Apicomplexan parasites (5). AP2XI-3 is believed to play a key role in tachyzoite replication (5).…”

“…Successful specific inhibition of AP2XI-3 through PPMO would be of considerable value because the antisense approach could then be used to abrogate transcription factors, which are difficult to target through other means. Transcription factors of interest are essential for sustaining tachyzoites and encysted bradyzoites such as AP2 transcription factors (5). Three transcription factors are important for maintaining the tachyzoite stage, and three transcription factors appear to be important for maintaining the bradyzoite stage.…”