Dihydrofolate reductase: thymidylate synthase, a bifunctional polypeptide from Crithidia fasciculata.

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Abstract
The MW of dihydrofolate reductase (EC 1.5.1.3) from protozoa was reported to be 5- to 10-fold larger than the isofunctional enzyme of most other organisms studied, based on gel filtration. This enzyme from the protozoal flagellate C. fasciculata was purified to homogeneity and was a bifunctional protein with thymidylate synthase (EC 2.1.1.45) activity. The purified protein, eluted from methotrexate-Sepharose columns by dihydrofolate, migrated as a single band on both nondenaturing and denaturing polyacrylamide gel electrophoresis. The monomer MW is 56,700 .+-. 200. The native MW was calculated to be 107,000 from a sedimentation coefficient of 5.9 and Stokes radius of 4.4 nm. Dihydrofolate reductase and thymidylate synthase activities of the rodent malaria organism Plasmodium berghei also copurified on Sephadex G-200 and methotrexate-Sepharose columns, suggesting that this unique bifunctional protein might occur throughout the Protozoa.