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6 Roles of Ribonuclease H in Reverse Transcription

James J. Champoux


The retroviral RNA genome is converted into double-stranded DNA by the process of reverse transcription. In the first step of reverse transcription, the genomic RNA with its associated transfer RNA primer serves as a template for the DNA polymerase activity of reverse transcriptase. Unlike the situation for all known single-stranded RNA viruses where the newly synthesized RNA strand is displaced from the template during synthesis, the newly synthesized minus DNA strand remains base-paired with the retroviral genome RNA. Since the DNA strand of the resulting DNA-RNA hybrid is subsequently used as a template for plus-strand synthesis, a mechanism must exist to unpair the DNA from the RNA. Equally important, continuation of minus-strand synthesis beyond the 5′ end of the genome depends on the unpairing of the DNA-RNA hybrid to allow base pairing of the nascent minus DNA strand with the complementary RNA sequence at the 3′ end of the genome (the first jump). In principle, the newly synthesized DNA strand could be separated from its RNA template by a helicase acting in conjunction with a single-strand-binding protein. Instead, retroviruses degrade the RNA moiety of the DNA-RNA hybrids using a ribonuclease H (RNase H) activity that is part of the reverse transcriptase protein (Mölling et al. 1971; for recent reviews, see Crouch 1990; Wintersberger 1990; Jacobo-Molina and Arnold 1991). The DNA polymerase and RNase H activities of reverse transcriptase appear to be the only enzymes required to produce the linear double-stranded DNA copy of the RNA genome that is subsequently integrated...

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