Multiple forms of human dihydrofolate reductase messenger RNA. Cloning and expression in Escherichia coli of their DNA coding sequence

The programming capacity for the synthesis of human dihydrofolic acid reductase in a rabbit reticulocyte lysate has been found to be greatly enhanced in the polysomal poly(A)-containing RNA from a methotrexate-resistant human cell variant (6A3), as compared to the RNA from its parental line (VA2-B). A major fraction of this promoting activity is associated with a 3.8 × 103 base RNA species detectable as a band in the ethidium bromide-stained electrophoretic pattern of the RNA from 6A3 cells, but not in the RNA from VA2-B cells. Furthermore, sucrose gradient fractionation experiments have indicated that another substantial portion of the messenger activity is associated with RNA components around 103 bases in size. Double-stranded complementary DNA synthesized from total poly(A)-containing RNA of 6A3 cells has been size fractionated, and both large (1400 to 3800 base-pairs) and small size complementary DNA (600 to 1400 base-pairs) species have been used separately to transform Escherich...