Unique progressive cleavage mechanism of HIV reverse transcriptase RNase H.

HIV-1 reverse transcriptase (RT) degrades the plus strand viral RNA genome while synthesizing the minus strand of DNA. Many RNA fragments, including the polypurine tracts, remain annealed to the new DNA. Several RTs are believed to bind after synthesis to degrade all RNA fragments except the polypurine tracts by a polymerization-independent mode of RNase H activity. For this latter process, we found that RT positions the RNase H active site approximately 18 nt from the 5' end of the RNA, making the primary cut. The enzyme rebinds or slides toward the 5' end of the RNA to make a secondary cut creating two products 8-9 nt long. RT then binds the new 5' end of the RNA created by the first primary or the secondary cuts to make the next primary cut. In addition, we observed another type of RNase H cleavage specificity. RT aligns the RNase H active site to the 3' end of the RNA, cutting 5 residues in. We determined the relative rates of these cuts, defining their temporal order. Results show that the first primary cut is fastest, and the secondary and 5-nt cuts occur next at similar rates. The second primary cuts appear last. Based on these results, we present a model by which RT progressively cleaves RNA fragments.