Pathways for dNTP biosynthesis in E. coli. RNR = ribonucleotide reductase, Ndk = Nucleoside diphosphate kinase, Dcd = dCTP deaminase, Dut = dUTPase, ThyA = Thymidylate synthase, Tmk = dTMP kinase. The pathway in parentheses along with the stippled paths indicates the minor pathway (~ 20%) for dUMP (and dTTP) synthesis. The major pathway for dTTP synthesis is via dCTP and dCTP deaminase.