17 Bacterial Reverse Transcriptase

Abstract

For most of the 20 years since the discovery of reverse transcriptase (RT), it was believed that these enzymes were restricted to eukaryotic cells. Although DNA polymerase I of Escherichia coli has RT activity (Loeb et al. 1971; Modak et al. 1973), i.e., this enzyme is able to use an RNA template to synthesize DNA in vitro, its RT activity does not function in the cell. In contrast, the primary function of eukaryotic RTs is to use RNA as a template to synthesize complementary DNA, which is then converted to double-stranded DNA by the same enzymes. Needless to say, RNA-directed cDNA synthesis is an obligatory step for retrotranscription and retroviral proliferation.

Until recently, no one was able to demonstrate the existence in any bacterial species of retrovirus-type RTs or other biological elements whose production requires cDNA synthesis. Important questions were left unanswered. What is the origin of RT? What role did RT play in the early steps of evolution? This question becomes particularly relevant if the current DNA world was preceded by the RNA world as proposed by Joyce (1989) and Waldrop (1989). Has RT activity evolved independently in separate lineages or has it been lost during the diversification of the eubacteria?

It now appears that the missing link between the eukaryotic RT and the putative primordial RT may be that bacterial RTs are evolutionarily related to eukaryotic RTs. The existence of bacterial RT was predicted in 1987 for the synthesis of a peculiar satellite single-stranded DNA (multicopy single-stranded DNA...