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 NADPH and FADH(2) as the reductant (PubMed:18493582). Is essential for growth of the pathogen on solid media in vitro; the essential function is something other than dTMP synthase (PubMed:12657046) (PubMed:22034487). (6R)-5,10-methylene-5,6,7,8-tetrahydrofolate + dUMP + H(+) + NADPH = (6S)-5,6,7,8-tetrahydrofolate + dTMP + NADP(+) Binds 4 FAD per tetramer. Each FAD binding site is formed by three monomers. Is potently inhibited by 5-fluoro-2'-deoxyuridine 5'-monophosphate (FdUMP), but in contrast to ThyA, is not inhibited by the folate-based 1843U89 (PubMed:18493582). A 5-alkynyl dUMP analog has been shown to highly inhibit ThyX (IC(50) value of 0.91 uM), while lacking activity against the classical mycobacterial thymidylate synthase ThyA, and therefore is a selective mycobacterial FDTS inhibitor (PubMed:21657202). kcat is 0.4 min(-1). Pyrimidine metabolism; dTTP biosynthesis. Homotetramer. Is expressed under the exponential phase of growth, and down-regulated upon starvation. Expression of thyX is significantly increased within murine macrophages or under acid stress. Is expressed at a lower level than thyA under all of the in vitro and in vivo growth conditions tested. Cells lacking this gene display impaired growth (PubMed:12657046). Strains with a thyX deletion could not be obtained (PubMed:22034487). Was identified as a high-confidence drug target. Crystallographic studies have shown that NADPH/NADP(+) binding expels both FAD and dUMP from the active site, by competing for the binding site (PubMed:16730023). However, the location of NADPH binding might not be biologically relevant (PubMed:18192395). Belongs to the thymidylate synthase ThyX family.