Reverse transcription—the reverse or “retro” flow of genetic information from RNA to DNA—is a hallmark of the retroviral replication cycle. The term “retroviruses” has now largely supplanted the earlier designation, “RNA tumor viruses,” recognizing that reverse transcription typifies this class of viruses better than any other property. The suggestion that some RNA viruses might replicate via a DNA intermediate that was subsequently incorporated into the host genome was first made by Howard Temin. By the 1960s, studies of bacteriophage had provided precedents for the integration of viral DNA into the host genome, and thus the most controversial aspect of Temin's proposal was the suggestion that the genome of an RNA virus could be converted into DNA. The stunning finding of an RNA-dependent DNA polymerase or reverse transcriptase (RT)—made independently by Temin and by Baltimore, who looked for and found the activity in purified virions—caught the scientific world off guard. It was then believed that reverse transcription flagrantly violated the central dogma of molecular biology—that genetic information always flowed from DNA to RNA to protein (Baltimore 1970; Temin and Mizutani 1970). The discovery that information could flow from RNA to DNA added considerable weight to Temin's earlier experiments which showed that the replication of retroviruses was sensitive to inhibitors of DNA synthesis. The ability of retroviruses to make a DNA copy of their genome and insert it into the genome of the host explains much of the biology of these viruses: their ability to establish persistent infections and to produce chronically infected cells, and the ability of a subset of retroviruses to transform infected cells rapidly and efficiently. Now, many years later, it is clear that RT is required for the replication of several classes of transposable elements and for certain plant and animal viruses. The AIDS pandemic has invigorated interest in RT because the enzyme plays a central part in retroviral replication and is an important therapeutic target (see Chapter 12. Although it is outside the scope of this chapter, it should be remembered that RT has had a critical role in the development of modern biology: The ability to convert RNA into DNA has been as important to molecular biologists as it has been to retrovirologists. It is worth taking a moment to reflect on how much more difficult it would have been to solve the puzzles posed by the organization and expression of genes in higher eukaryotes if RT did not exist.

A number of reviews that focused specifically on RT or include RT as a major theme have recently appeared (see, e.g., Varmus and Brown 1989; Goff 1990; Whitcomb and Hughes 1992; Skalka and Goff 1993). This chapter focuses on an understanding of the process of reverse transcription in the retroviral life cycle and on ways in which the properties of retroviral RT relate to retroviral replication.