ABSTRACT
Sequence analysis of a cDNA encoding dihydrofolate reductase (DHFR) from a selected methotrexate-resistant Drosophila melanogaster cell line (S3MTX) revealed a substitution of Gln for Leu at position 30. Although the S3MTX cells were approximately 1000 fold more resistant to methotrexate (MTX), the karyotype was similar to the parental line and did not show elongated chromosomes. Furthermore, kinetic analysis of the recombinant enzyme showed a decreased affinity for MTX by the mutant DHFR. To determine if the resistance phenotype could be attributed to the mutant allele, Drosophila Dhfr cDNAs isolated from wild type and S3MTX cells were expressed in Chinese hamster ovary (CHO) cells lacking endogenous DHFR. The heterologous insect DHFRs were functional in transgenic clonal cell lines, showing approximately 400-fold greater MTX resistance in the cell line transfected with the mutant Dhfr than the wild type Dhfr. Resistance to other antifolates in the CHO cells was consistent with the drug sensitivities seen in the respective Drosophila cell lines. ELevated Levels of Dhfr transcript and DHFR in transgenic CHO cells bearing the mutant cDNA were not seen. Taken together, these results demonstrate that a single substitution in Drosophila DHFR alone can confer Levels of MTX resistance comparable with that observed after considerable gene amplification in mammalian cells.