Dihydrofolate reductase (DHFR) and thymidylate synthase (TS) activities from cell suspension cultures of Daucus carota were shown to copurify on (NH4)2S04 fractionation, DEAE Sephadex and methotrexate- Sepharose affinity chromatography and to share approximately the same M, (183 kDa and 185 kDa respectively) as judged by gel filtration on Sephacryl S-200. The copurified protein migrated as a single band on polyacrylamide gel electrophoresis under denaturing conditions. Both activities could be eluted from the same position of the native gel. Moreover, methotrexate-resistant cell lines which overproduce DHFR revealed to have a parallel higher level of TS. It is therefore proposed and discussed that in carrot, similarly to protozoa, TS and DHER are present on a single bifunctional polypeptide of 58 kDa.
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