14 Retrotransposon Reverse Transcription

Abstract

During the past decade, it has become evident that genes encoding putative reverse transcriptases (RTs) are widespread genomic residents. Potential RT-coding domains have been identified in genes inhabiting a variety of genomic niches within animal, plant, protozoan, fungal, and even eubacterial cells. The existence of extensive sequence data has led to a number of studies of the evolutionary relationships among RT-encoding elements and speculations about the origin of this enzyme in a pre-DNA world, preceding the prokaryotic/eukaryotic divergence (Temin 1985; Darnell and Doolittle 1986; Wintersberger and Wintersberger 1987; Varmus 1989). This chapter concentrates on the available evidence for the existence of RTs encoded by three broad groups of transposable elements as well as evidence suggesting that these elements transpose through an RNA-mediated mechanism. These three groups of elements, collectively called “retrotransposons,” differ from the much more extensively studied retroviruses in a number of ways, most conspicuously by their apparent inability to be transferred horizontally. Given the impossibility of discussing all of the large number of sequenced transposons thought to encode RT, this chapter concentrates on those systems that have proved to be most amenable to experimental dissection of the detailed mechanisms involved in transposon reverse transcription. Recent comprehensive reviews of these genomic elements can be found elsewhere (Boeke 1988; Berg and Howe 1989; Echalier 1989; Boeke and Sandmeyer 1991), and more specialized reviews are referenced in the text under specific topics. A list of structural features of the elements discussed in this chapter is presented in Table 1.

The wide...