Catalyzes the reductive methylation of 2'-deoxyuridine-5'-monophosphate (dUMP) to 2'-deoxythymidine-5'-monophosphate (dTMP) while utilizing 5,10-methylenetetrahydrofolate (mTHF) as the methyl donor and reductant in the reaction, yielding dihydrofolate (DHF) as a by-product. This enzymatic reaction provides an intracellular de novo source of dTMP, an essential precursor for DNA biosynthesis. (6R)-5,10-methylene-5,6,7,8-tetrahydrofolate + dUMP = 7,8-dihydrofolate + dTMP Is potently inhibited by 5-fluoro-2'-deoxyuridine 5'-monophosphate (FdUMP), and by the folate-based 1843U89. kcat is 18 min(-1). Pyrimidine metabolism; dTTP biosynthesis. Homodimer. Is expressed under the exponential phase of growth, and down-regulated upon reaching the stationary phase or under both oxidative and nitrosative stress. Expression of thyA is significantly increased within murine macrophages or under acid stress. Is expressed at a higher level than thyX under all of the in vitro and in vivo growth conditions tested. Cells lacking this gene display an in vitro growth defect, but deletion of thyA does not affect M.tuberculosis in vivo growth. Moreover, thyA deletion confers PAS resistance. Mutations within this gene have been associated with resistance to p-aminosalicylic acid (PAS) in some PAS-resistant M.tuberculosis clinical isolates and spontaneous mutants (PubMed:19237648) (PubMed:15225321). It seems that functional ThyA is required for M.tuberculosis to be sensitive to PAS (PubMed:22034487). Belongs to the thymidylate synthase family. Bacterial-type ThyA subfamily. Truncated N-terminus.