5 The Fidelity of Retroviral Reverse Transcriptases

Abstract

The reported rate at which mutations are generated in retroviruses ranges from 10⁻⁶ to 10⁻⁴ mutations per nucleotide per cycle through a host cell (Batschelet 1976; Leider et al. 1988; Pathak and Temin 1990). Hyper-mutagenesis in spleen necrosis virus (SNV) (Pathak and Temin 1990) is one special case where the rate is even higher, 10⁻². Such mutation rates are considerably higher than those of DNA-based microbes (Drake 1991), which range from 10⁻⁷ to 10⁻¹¹ mutations per nucleotide per generation. Analogous to the situation in DNA-based organisms, retroviral diversity presumably results from several competing forces (Drake 1991), including the disadvantage of deleterious mutations, the advantage of rapid adaptation to a changing environment in the host, and the cost in terms of the energy required to avoid or eliminate mutations.

Three different replication systems operate during the life cycle of a retrovirus. The reverse transcriptase (RT) polymerizes deoxyribonucleotides, first using viral RNA and then the newly made complementary strand DNA as a template. Cellular DNA polymerases replicate the integrated viral DNA. Finally, RNA polymerase II transcribes the proviral DNA into RNA genomes that are packaged into virions. Although it is possible for mutations to be introduced into the viral genome during any one of these replication steps, much attention has focused on the subject of this chapter, i.e., the fidelity of DNA synthesis catalyzed by RTs. We begin this review by describing the steps that determine fidelity during DNA polymerization reactions, and then we consider several models for how DNA polymerases make...