Is able to catalyze the biosynthesis of dTMP using dUMP, tetrahydrofolate and formaldehyde in vitro, i.e. a reaction equivalent to that catalyzed by bacterial thymidylate synthases (EC 2.1.1.45). However, M.jannaschii like most methanogenic Archaea lacks folates, thus the physiological cosubstrate is unknown but is likely one of the non-methylated methanopterin biosynthetic intermediates. Pyrimidine metabolism; dTTP biosynthesis.