Characterization of a Mutant ofToxoplasma gondiiResistant to Aphidicolin1

Abstract

Aphidicolin, a mycotoxin that inhibits eukaryotic DNA polymerase alpha, blocked the growth of T. gondii in confluent cultured human fibroblasts. Aphidicolin immediately inhibited DNA synthesis by T. gondii, it had a delayed and less dramatic effect on RNA synthesis. A mutant of T. gondii resistant to aphidicolin was isolated with the aid of mutagenesis by ethylnitrosourea. Parasite growth measured 3 days after drug treatment and parasite DNA synthesis measured immediately after drug treatment were, respectively, 5- and 4-fold more resistant to aphidicolin in the mutant as compared with the wild type parasite. The mutant had a 3-fold greater capacity than the wild type to incorporate uracil into its deoxycytidine triphosphate pool. This increased deoxycytidine triphosphate pool is the probable explanation for the mutant''s resistance because this deoxynucleotide is known, in mammalian cells, to reverse the inhibition of DNA synthesis by aphidicolin in a competitive manner.