May catalyze the biosynthesis of dTMP using an unknown cosubstrate. In vitro, also catalyzes the dehalogenation of 5-bromo-deoxyuridine monophosphate (Br-dUMP) and the tritium exchange of [5-3H]deoxyuridine monophosphate ([5-3H]dUMP). Optimum pH is 9.0 with [5-3H]dUMP as substrate, and 7.0 with Br-dUMP as substrate. Optimum temperature is 70 degrees Celsius with [5-3H]dUMP as substrate, and 50-70 degrees Celsius with Br-dUMP as substrate. Heating the protein for 10 min at 100 degrees Celsius abolished the activity. Pyrimidine metabolism; dTTP biosynthesis. Monomer. Belongs to the thymidylate synthase family. Archaeal-type ThyA subfamily.