Identification of TgENT1 as the TgUUT1 Uracil/Uridine Transporter of <em>Toxoplasma gondii</em>

The protozoan pathogen Toxoplasma gondii is responsible for toxoplasmosis a disease that can be deadly in immunocompromised patients and the developing fetus during pregnancy. Current treatments are widely considered to be suboptimal. We have recently reported that 5-fluoropyrimidines have highly promising anti-toxoplasmosis effects and are taken up by the parasite by a high affinity uracil/uridine transporter, TgUUT1. Here, we attempt to identify the gene encoding this transport protein. The only nucleoside or nucleobase family identified in the T. gondii genome was the Equilibrative Nucleoside Transporter (ENT) family, with four members. Of these, TgAT1 is known to be purine-specific, and deletion of the TgENT2 and TgENT3 genes, either separately or jointly, did not affect uridine transport or sensitivity to 5-fluoropyrimidines. In contrast, depletion of TgENT1, an essential gene, resulted in significant reduction in the uptake of both uracil and uridine but not of the amino acid tryptophan. Moreover, expression of TgENT1 in a Leishmania mexicana cell line with low endogenous uracil uptake rates significantly increased uracil uptake for these cells. We conclude that it is highly probable that TgENT1 encodes the T. gondii uracil/uridine transporter.