Toxoplasma gondii: The biochemical basis of resistance to emimycin

Abstract

Emimycin was a potent and selective inhibitor of the growth and nucleic acid synthesis of Toxoplasma gondii in human fibroblasts. An emimycin-resistant mutant of T. gondii lost the pyrimidine salvage enzyme uracil phosphoribosyltransferase, the same enzyme absent in parasites resistant to fluorodeoxyuridine. The mutant resistant to emimycin was completely cross-resistant to fluorodeoxyuridine. Emimycin was as good a substrate as uracil for the uracil phosphoribosyltransferase of T. gondii. [ 3H]Emimycin supplied in the medium of cultures with actively growing intracellular parasites was converted to emimycin riboside-5′-phosphate in the soluble pool of T. gondii. All other emimycin analogs of uracil-containing nucleotides were also formed but little emimycin riboside diphosphate- N-acetylhexosamine was found. [ 3H]Emimycin was not converted to analogs of the cytidine nucleotides. When intracellular T. gondii were treated with a concentration of [ 3H]emimycin that partially inhibited parasite RNA synthesis, much less [ 3H]emimycin was incorporated into RNA than would be predicted by the amount of intracellular [ 3H]emimycin riboside triphosphate.