A CTP Synthase Undergoing Stage-Specific Spatial Expression Is Essential for the Survival of the Intracellular Parasite .

Cytidine triphosphate synthase catalyzes the synthesis of cytidine 5'-triphosphate (CTP) from uridine 5'-triphosphate (UTP), the final step in the production of cytidine nucleotides. CTP synthases also form filamentous structures of different morphologies known as , whose functions in most organisms are unknown. Here, we identified and characterized a novel CTP synthase (CTPS) from . We show that CTPS is capable of substituting for its counterparts in the otherwise lethal double mutant (Δ Δ) of . Equally, recombinant CTPS purified from encodes for a functional protein in enzyme assays. The epitope-tagged CTPS under the control of its endogenous promoter displays a punctate cytosolic distribution, which undergoes spatial reorganization to form foci or filament-like structures when the parasite switches from a nutrient-replete (intracellular) to a nutrient-scarce (extracellular) condition. An analogous phenotype is observed upon nutrient stress or after treatment with a glutamine analog, 6-diazo-5-oxo-L-norleucine (DON). The exposure of parasites to DON disrupts the lytic cycle, and the CTPS is refractory to a genetic deletion, suggesting an essential requirement of this enzyme for . Not least, this study, together with previous studies, supports that CTP synthase can serve as a potent drug target, because the parasite, unlike human host cells, cannot compensate for the lack of CTP synthase activity.