DNA amplification in antifolate-resistant Leishmania. The thymidylate synthase-dihydrofolate reductase gene and abundant mRNAs.

Leishmania tropica promastigotes selected for resistance to the dihydrofolate reductase inhibitor, methotrexate, or the thymidylate synthase inhibitor, 5,8-dideaza-10-propargyl folate, overproduce a bifunctional thymidylate synthase-dihydrofolate reductase and possess a 30-kilobase region of amplified DNA. Five fragments, resulting from BglII digestion of this amplified DNA, were cloned into vectors and utilized as probes to examine mRNA in these organisms. Four mRNA species which hybridize to the amplified DNA sequences were found in both resistant and wild-type Leishmania, but were about 40-fold more abundant in the drug-resistant cells. Three of the four mRNAs are transcribed from the same strand of DNA, are clustered, and appear to have partial overlapping sequences. The thymidylate synthase-dihydrofolate reductase gene was localized to a specific region of the amplified unit of DNA by hybridization with mouse cDNA containing thymidylate synthase sequences and with a synthetic oligonucleotide 41 nucleotides in length, prepared on the basis of the partial amino acid sequence of the Leishmania enzyme. Furthermore, mRNA hybrid-selected using a plasmid containing sequences of the putative gene was shown to direct in vitro synthesis of the bifunctional protein.