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

The molecular weight of dihydrofolate reductase (5,6,7,8-tetrahydrofolate:NADP+ oxidoreductase, EC 1.5.1.3) from protozoa has been 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 Crithidia fasciculata has been purified to homogeneity and found to be a bifunctional protein with thymidylate synthase (5,10-methylene tetrahydrofolate:dUMP C-methyltransferase, 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 Mr is 56,700 +/- 200. The native Mr 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 methotexate-Sepharose columns, suggesting that this unique bifunctional protein might occur throughout the Protozoa.