Mutagenesis by human immunodeficiency virus reverse transcriptase: incorporation of O6-methyldeoxyguanosine triphosphate.

The high frequency of incorporation of non-complementary nucleotides by HIV-1 reverse transcriptase is likely to be a major factor in the exceptionally rapid accumulation of viral mutations during the course of AIDS infections. To investigate whether this high level of infidelity is also associated with the incorporation of nucleotide analogs, we analyzed O6-methyldeoxyguanosine triphosphate and compared the incorporation of this analog by HIV-1 reverse transcriptase to that catalyzed by other DNA synthesizing enzymes. Our results indicate that O6-methyldeoxyguanosine triphosphate serves as a substrate for DNA synthesized in vitro by HIV-1 RT on both DNA and RNA templates. The product DNA contains the modified purine; it is sensitive to the repair enzyme, O6-methylguanine methyltransferase, which specifically reacts with DNA containing methylated guanines at the O6 position. Using a forward mutation assay we demonstrated that the nucleotide analog incorporated by HIV-1 RT is mutagenic. The mutations produced are single-base substitutions opposite template thymidines and result in A:T --> G:C transitions. The incorporation of a mutagenic nucleotide by HIV-1 RT highlights the possibility of increasing the rate of mutagenesis of HIV by the use of nucleotides that form non-complementary base pairs at high frequency.