5-Fluorouracil augmentation of dihydrofolate reductase gene transcripts containing intervening sequences in methotrexate-resistant KB cells.

Several fragments of the human dihydrofolate reductase gene (tetrahydrofolate dehydrogenase, 5,6,7,8-tetrahydrofolate NADP+ oxidoreductase, EC 1.5.1.3) were isolated from gene-amplified KB7B cells and characterized. Recombinant plasmids containing intron sequences were constructed. Probes prepared from these plasmids were tested for dihydrofolate reductase precursor mRNA specificity via solution hybridization studies and Northern blot analysis. One probe, p0.69EH, was shown to be specific for dihydrofolate reductase RNA by its greatly enhanced level of hybridization with total RNA from dihydrofolate reductase gene-amplified versus non-amplified cells. In addition, solution hybridization studies with various classes of RNA and Northern blot analysis revealed that p0.69EH hybridizes predominantly with polyadenylated, high molecular weight, nuclear RNA species. Subsequent solution hybridization studies revealed a disproportionate 5-fluorouracil-induced increase in dihydrofolate reductase intron-containing RNA over dihydrofolate reductase mRNA. These results suggest that 5-fluorouracil incorporation into RNA may inhibit the conversion of precursor mRNA to mature mRNA.