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tRNAs as Primers for Reverse Transcriptases

James E. Dahlberg


RNA-directed DNA polymerases (reverse transcriptases) are the enzymes responsible for synthesis of the DNA copy of RNA tumor virus genomic RNA. The reverse transcriptases associated with avian sarcoma/leukosis viruses (ASV/ALV) differ from the enzymes of the murine viruses, such as murine leukemia virus (MuLV) (for reviews, see Temin 1971; Temin and Baltimore 1972; Verma 1977; Taylor 1977; Bishop 1978). Nevertheless, the enzymes from these two sources share many important features. They are both coded for by their respective RNA genomes and are present in mature virus particles. After virus infection of susceptible cells, the enzymes initiate synthesis of a cDNA (complementary DNA) copy of the virion genomic RNA. Like other DNA polymerases, reverse transcriptases are unable to initiate DNA synthesis de novo; rather, they add deoxyribonucleoside triphosphates to preexisting primer RNAs. The primer RNAs are present in virions, tightly associated with the template, genomic RNA (Verma et al. 1971; Duesberg et al. 1971; Canaani and Duesberg 1972; Faras et al. 1973b).

Identification and analysis of the RNA primers is greatly facilitated by the fact that the virions of RNA tumor viruses contain all of the macromolecular components necessary for DNA synthesis. These include the primer RNAs, the viral coded enzymes, and the RNA template, which is the viral genome. This genome is a 70S RNA dimer consisting of two identical subunits of 35S RNA, each of which is 8000–10,000 nucleotides long (Fan and Paskind 1974; Billeter et al. 1974; Beemon et al. 1976). After disruption...

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