Plant Physiol.
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Plant Physiology 91:1168-1173 (1989)
© 1989 American Society of Plant Biologists

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Metabolism and Enzymology

Coregulation of Dihydrofolate Reductase and Thymidylate Synthase in Overproducer Cell Lines of Wild Carrot 1

Gabor Lazar, Ildiko Toth, Letitia Haakonsen and Howard M. Goodman

Department of Genetics, Harvard Medical School, Boston, Massachusetts 02114, Department of Molecular Biology, Massachusetts General Hospital, Boston, Massachusetts 02114

Dihydrofolate reductase (DHFR) and thymidylate synthase (TS) activities are associated with a 285,000 molecular weight enzyme complex in carrot (Daucus carota L.). Selection for methotrexate (MTX) resistance by stepwise increase of the concentration of MTX results in a high frequency adaptation to MTX with little or no significant increase in DHFR activity. However, when as a second step following MTX selection a specific inhibitor of TS, 5-fluoro-2-deoxyuridine was used, DHFR overproducer lines were obtained. The overproduction phenotype of the lines was almost completely lost after 8 weeks of growth in the absence of selection pressure. Although DHFR and TS are independent gene products, their activities increase in proportion (~20-fold) in the overproducer lines. This strongly suggests that DHFR and TS are not only functionally and physically linked in the same enzyme complex, but also are coregulated. These cell lines resemble the MTX-induced DHFR overproducer amplified cell lines of mammalian origin in their mode of selection, high frequency of appearance, elevated enzyme activity, and increased specific mRNA levels.

1 This work was supported by a grant from Hoechst AG.






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