SUMMARY Apicomplexan parasites are leading causes of human and livestock diseases—like malaria and toxoplasmosis—yet most of their genes remain uncharacterized. Here, we present the first genome-wide genetic screen of an apicomplexan. We adapted CRISPR/Cas9 to assess the contribution of each gene from the human parasite Toxoplasma gondii during infection of fibroblasts. Our analysis defines ~200 previously uncharacterized, fitness-conferring genes unique to the phylum, from which 16 were investigated, revealing essential functions during infection of human cells. Secondary screens identify as an invasion factor the claudin-like apicomplexan microneme protein (CLAMP), which resembles mammalian tight-junction proteins and localizes to secretory organelles, making it critical to the initiation of infection. CLAMP is present throughout sequenced apicomplexan genomes, and is essential during the asexual stages of the malaria parasite Plasmodium falciparum. These results provide broad-based functional information on T. gondii genes and will facilitate future approaches to expand the horizon of antiparasitic interventions.

Synthetic posttranscriptional regulation of gene expression is important for understanding fundamental biology and programming new cellular processes in synthetic biology. Previous strategies for regulating translation in eukaryotes have focused on disrupting individual steps in translation, including initiation and mRNA cleavage. In emphasizing modularity and cross-organism functionality, these systems are designed to operate orthogonally to native control mechanisms. Here we introduce a broadly applicable strategy for robustly controlling protein translation by integrating synthetic translational control via a small-molecule-regulated RNA–protein module with native mechanisms that simultaneously regulate multiple facets of cellular RNA fate. We demonstrate that this strategy reduces ‘leakiness' to improve overall expression dynamic range, and can be implemented without sacrificing modularity and cross-organism functionality. We illustrate this in Saccharomyces cerevisae and the non-model human malarial parasite, Plasmodium falciparum. Given the limited functional genetics toolkit available for P. falciparum, we establish the utility of this strategy for defining essential genes.

SUMMARY New antimalarial drugs are urgently needed to control drug resistant forms of the malaria parasite, Plasmodium falciparum. Mitochondrial electron transport is the target of both existing and new antimalarials. Herein, we describe 11 genetic knockout (KO) lines that delete six of the eight mitochondrial tricarboxylic acid (TCA) cycle enzymes. Although all TCA KOs grew normally in asexual blood stages, these metabolic deficiencies halted lifecycle progression in later stages. Specifically, aconitase KO parasites arrested as late gametocytes, whereas α-ketoglutarate dehydrogenase deficient parasites failed to develop oocysts in the mosquitoes. Mass spectrometry analysis of 13C isotope-labeled TCA mutant parasites showed that P. falciparum has significant flexibility in TCA metabolism. This flexibility manifested itself through changes in pathway fluxes and through altered exchange of substrates between cytosolic and mitochondrial pools. Our findings suggest that mitochondrial metabolic plasticity is essential for parasite development.

Genetic manipulation of Plasmodium falciparum in culture through transfection has provided numerous insights into the molecular and cell biology of this parasite. The procedure is rather cumbersome, and is limited by the number of drug-resistant markers that can be used for selecting transfected parasites. Here we report a new selectable marker that could allow multiple transfections. We have taken advantage of our finding that a critical function of the mitochondrial electron transport chain (mtETC) in the erythrocytic stages of P. falciparum is the regeneration of ubiquinone as co-substrate of dihydroorotate dehydrogenase (DHODH), and that transgenic P. falciparum expressing ubiquinone-independent DHODH from yeast (yDHODH) are resistant to all mtETC inhibitors. We assessed the possibility of using yDHODH as a positive selectable marker for transfections of P. falciparum, including its use in gene disruption strategies. We constructed a transfection vector designed for gene disruption, termed pUF-1, containing the yDHODH gene as the positive selection marker in combination with a previously described fused yeast cytosine deaminase-uracil phosphoribosyl transferase gene as a negative selection marker. Transfection of the D10 strain followed by selection with atovaquone yielded positively selected parasites containing the plasmid, demonstrating that yDHODH can be used as a selective marker. Atovaquone, however, could not be used for such selection with the Dd2 strain of P. falciparum. On the other hand, we demonstrated that yDHODH transgenic parasites could be selected in both strains by Plasmodium DHODH-specific triazolopyrimidine-based inhibitors. Thus, selection with DHODH inhibitors was superior in that it successfully selected transgenic Dd2 parasites, as well as yielded transgenic parasites after a shorter period of selection. As a proof of concept, we have successfully disrupted the type II vacuolar proton-pumping pyrophosphatase gene (PfVP2) in P. falciparum by double crossover recombination, showing that this gene is not essential for the survival of blood stage parasites.

SUMMARY The malaria parasite exports numerous proteins into its host red blood cell (RBC). The trafficking of these exported effectors is complex. Proteins are first routed through the secretory system, into the parasitophorous vacuole (PV), a membranous compartment enclosing the parasite. Proteins are then translocated across the PV membrane in a process requiring ATP and unfolding. Once in the RBC compartment the exported proteins are then refolded and further trafficked to their final localizations. Chaperones are important in the unfolding and refolding processes. Recently, it was suggested that the parasite TRiC chaperonin complex is exported, and that it is involved in trafficking of exported effectors. Using a parasite-specific antibody and epitope-tagged transgenic parasites we could observe no export of Plasmodium TRiC into the RBC. We tested the importance of the parasite TRiC by creating a regulatable knockdown line of the TRiC-θ subunit. Loss of the parasite TRiC-θ led to a severe growth defect in asexual development, but did not alter protein export into the RBC. These observations indicate that the TRiC proteins play a critical role in parasite biology, though their function, within the parasite, appears unrelated to protein trafficking in the RBC compartment.