Reverse transcription is the process whereby the single-stranded RNA genome of a retrovirus is converted into double-stranded DNA by the DNA polymerase and ribonuclease H (RNase H) activities of virus-coded reverse transcriptase (RT) (2 ,16 ). The former activity of reverse transcriptase has been studied extensively, both as a potential target for antiviral therapy and a tool for molecular biology. However, increasing attention has been directed recently toward understanding the roles of RNase H activity, both nonspecific and highly specialized, in reverse transcription (reviewed inref.10 ). A complete summary of the process is depicted in Fig. 1. Quantitatively, the most important RNase H function is to degrade the RNA strand of ∼10,000 bp RNA-DNA replication intermediate, eliminating the need for strand-displacement during synthesis of the second or (+) strand. This degradation is largely nonspecific, yet several cleavage events demand a high degree of precision, such as scission of the RNA strand at specific positions near the 3− termini of small, purine-rich stretches of RNA called polypurine tracts (PPTs). These RNAs serve as primers for (+) strand synthesis, but must also be removed by the enzyme once initiation has occurred. The (−;) strand primer, a host-derived tRNA, is removed after its 3′-terminus is copied during (+) strand synthesis. If the (+) and (−) strand primers are not precisely removed, the resulting errors would alter the 5′- and 3′-termini of viral DNA (Fig. 1E). This would inhibit integration of viral DNA into the host-cell genome and perhaps adversely affect the viability of the virus at later stages of its life cycle.
