RNase H requirements for the second strand transfer reaction of human immunodeficiency virus type I reverse transcription

Retroviral reverse transcriptase (RT) enzymes are responsible for transcribing viral RNA into double-stranded DNA. An in vitro assay to analyze the second strand transfer event during human immunodeficiency virus type 1 (HIV-1) reverse transcription has been developed. Model substrates were constructed which mimic the vital intermediate found during plus-strand strong-stop synthesis. Utilizing wild-type HIV-1 RT and a mutant E478Q RT, the requirement for RNase H activity in this strand transfer event was analyzed. In the presence of Mg²⁺, HIV-1 RT was able to fully support the second strand transfer reaction in vitro. However, in the presence of Mg²⁺, the E478Q RT mutant had no detectable RNase H activity and was unable to support strand transfer. In the presence of Mn²⁺, the E478Q RT yields the initial endoribonucleolytic cleavage at the penultimate C residue of the tRNA primer yet does not support strand transfer. This suggests that subsequent degradation of the RNA primer by the RNase H domain was required for strand transfer. In reactions in which the E478Q RT was complemented with exogenous RNase H enzymes, strand transfer was supported. Additionally, we have shown that HIV-1 RT is capable of supporting strand transfer with substrates that mimic tRNA(His) as well as the authentic tRNA₃/(Lys).