Stable transformation of Toxoplasma gondii based on a pyrimethamine resistant trifunctional dihydrofolate reductase-cytosine deaminase-thymidylate synthase gene that confers sensitivity to 5-fluorocytosine

To improve genetic models available for the analysis of apicomplexan protoz oan parasites, bacterial sequences encoding the 427 amino acid cytosine dea minase (CD) gene were fused, in-frame, to an engineered linker domain of th e high level pyrimethamine resistant form of the parasite bifunctional dihy drofolate reductase-thymidylate synthase (DHFR-TS) gene. Toxoplasma gondii was transformed with the plasmid containing the fused pyrimethamine resista nt dihydrofolate reductase-cytosine deaminase-thymidylate synthase (DHFRm2m 3-CD-TS) gene and parasites were selected in a high level of pyrimethamine. Transfected parasites that acquired resistance to pyrimethamine were clone d and evaluated for expression of the CD genetic marker. CD transgenic para sites acquired a high sensitivity to 5-fluorocytosine due to the intraparas itic conversion of this non-toxic prodrug to the cytotoxic compound 5-fluor ouracil. Exogenously supplied cytosine or uracil rescued the growth of CD t ransgenic T. gondii parasites that were cultured in the presence of cytotox ic concentrations of 5-fluorouracil or 5-fluorocytosine. Bacterial CD fused to the pyrimethamine resistant DHFR-TS marker provides a novel genetic too l for new positive and negative genetic selection strategies in several pro tozoan parasites. An advantage of the CD genetic marker is that it is deriv ed from a bacterial gene and can therefore be used in nearly any parasite g enetic background for negative selection. This novel system should facilita te new approaches for the development of improved model genetic systems for the biological investigation of apicomplexan parasites. (C) 1999 Elsevier Science B.V. All rights reserved.