Fig. 1.

Mechanisms for FDTS catalyzed methylene transfer. (A) A recently proposed mechanism for FDTS enzymes involving direct transfer of the methylene between CH₂H₄folate and dUMP (4, 8). (B) A mechanism proposed for methylene transfer that involves an enzymatic arginine residue (16). Please note that the methylene transfer is the focus of this figure, and the methylene could in principle be transfered to the reduced dUMP (as drawn), to the oxidized dUMP as proposed in ref. 8, or to dUMP activated by Michael addition of Ser to C6, as originally suggested by ref. 16. R, 2′-deoxyribose-5′-phosphate; R′, (p-aminobenzoyl)-glutamate.