Expression of the plasmid-encoded type I dihydrofolate reductase gene in cultured mammalian cells: a novel selectable marker |
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Authors: | C S Simonsen M Walter A D Levinson |
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Affiliation: | Department of Molecular Biology, Genentech, Inc., San Francisco, CA 94080. |
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Abstract: | A recombinant plasmid has been designed to express the gene encoding a type I methotrexate-resistant dihydrofolate reductase, derived from the bacterial plasmid R483, in DHFR- Chinese hamster ovary cells. Vectors containing the wild type gene, whose coding sequence initiates with a GTG codon, fail to direct the synthesis of detectable levels of protein. Substitution of the GTG codon by an AG codon using in vitro mutagenesis overcomes this block; cells transfected with the modified vector synthesize a functional prokaryotic protein that sustains the growth of these cells in the presence of dihydrofolic acid in the culture media. This property is consistent with the inability of the type I enzyme to reduce folate to dihydrofolate, and enabled the development of a selection strategy whereby prokaryotic and mammalian DHFRs genes could be used sequentially as independently selectable markers. |
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