2'-Deoxy-6-thioguanosine 5'-triphosphate (S6dGTP), a metabolite of the antileukemia agent 6-thioguanine, was evaluated as a substrate for purified human DNA polymerases. Using bacteriophage M13 single-strand DNA as a template, S6dGTP substituted efficiently for dGTP and stimulated DNA synthesis in reactions without dGTP, with DNA polymerases alpha, delta, and gamma from the human leukemia cell line K562. The apparent Km values for dGTP and S6dGTP were very similar, i.e., 1.2 microM each for polymerase alpha, 2.8 and 3.6 microM, respectively, for polymerase delta, and 0.8 microM each for polymerase gamma; however, the relative Vmax values for the modified nucleotide were 25-50% lower than those of the corresponding natural substrate. Using a highly sensitive electrophoretic assay of chain elongation across M13mp9 (+)-strand DNA by the aforementioned human DNA polymerases, S6dGTP was shown to be incorporated at the 3' end of the nascent growing DNA chain, and the patterns of chain extension with S6dGTP as substrate were identical to those obtained in the presence of dGTP. There were no major differences using S6dGTP in place of dGTP with these DNA polymerases; however, at higher concentrations (1-10 microM) the analog stimulated primer elongation in reactions without dATP, indicating some misincorporation at sites of S6G.T base pairs during DNA synthesis. Using p(dA)12-18 as the initiator for calf thymus terminal deoxynucleotidyltransferase, S6dGTP inhibited the incorporation of all four natural deoxyribonucleoside 5'-triphosphates into the primer, in a competitive manner. The apparent Ki values for the analog were 6-20 times lower than the Km values for the four endogenous substrates. As a substrate, S6dGTP was added to the 3'-hydroxyl termini of primer, although tailing efficiency with the analog was lower than that in the presence of the natural substrate. These findings indicate that S6dGTP is a relatively good substrate for several mammalian DNA polymerases, including terminal deoxynucleotidyltransferase.