(6R)-5,10-methylene-5,6,7,8-tetrahydrofolate + dUMP = 7,8-dihydrofolate + dTMP Pyrimidine metabolism; dTTP biosynthesis. Homodimer. This enzyme is also expressed by the thyA gene of E.coli; the phage and host synthases exhibit striking dissimilarities in both structure and function. Belongs to the thymidylate synthase family.