Thymidylate synthase required for de novo biosynthesis of pyrimidine deoxyribonucleotides. Required for both nuclear and mitochondrial DNA synthesis. (6R)-5,10-methylene-5,6,7,8-tetrahydrofolate + dUMP = 7,8-dihydrofolate + dTMP Inhibited by 5-fluoro-2'-deoxyuridine 5'-monophosphate (FdUMP). Optimum pH is 6.8-7.2. Pyrimidine metabolism; dTTP biosynthesis. Homodimer. Localizes to the nuclear periphery. Transcribed in a periodic manner during the cell cycle with maximal mRNA levels occurring just prior to the onset of DNA replication. The promoter contains 8-base-pair motifs (ACGCGTNA) called the MluI cell-cycle boxes (MCBs), which mediate transcription regulation by SWI6. Belongs to the thymidylate synthase family.