Selective action of 3′-azido-3′-deoxythymidine 5′-triphosphate on viral reverse transcriptases and human DNA polymerases

The action of 3′-azido-3′-deoxythymidine 5′-triphosphate (N₃dTTP) on DNA strand elongation catalyzed by human immunodeficiency virus type 1 reverse transcriptase was evaluated in comparison with human DNA polymerase α and proliferating cell nuclear antigen-independent DNA polymerase δ. Sequencing gel analysis demonstrated that the human immunodeficiency virus 1 reverse transcriptase preferentially incorporated N₃dTTP into the T sites of the growing DNA strands and caused chain termination in a dose-dependent manner. This effect was observed even when the N₃dTTP concentration was 0.3 μM, 100-fold less than dTTP. Studies with reverse transcriptases from avian myeloblastosis virus and Moloney murine leukemia virus showed that N₃dTTP was also efficiently incorporated into DNA by these enzymes and terminated DNA strand elongation. In contrast, human DNA polymerases α and δ did not incorporate detectable amounts of N₃dTTP into the DNA and were not inhibited by 300 μM N₃dTTP. The selective incorporation of the chain-terminating nucleotide by the viral reverse transcriptases appears to be a molecular basis for the positive therapeutic index of 3′-azido-3′-deoxythymidine.