End-to-End Template Jumping by the Reverse Transcriptase Encoded by the R2 Retrotransposon*

The reverse transcriptase encoded by the non-long terminal repeat retrotransposon R2 has been shown to be able to jump from the 5′-end of one RNA template (the donor) to the 3′-end of a second RNA template (the acceptor) in the absence of preexisting sequence identity between the two templates. These jumps between RNA templates have similarity to the end-to-end template jumps described for the RNA-directed RNA polymerases encoded by certain RNA viruses. Here we describe for the first time the mechanism by which such end-to-end template jumps can occur. Most template jumps by the R2 reverse transcriptase are brought about by the enzyme's ability to add nontemplated (overhanging) nucleotides to the cDNA when it reaches the end of the donor RNA. The enzyme then anneals these overhanging nucleotides to sequences at the 3′-end of the acceptor RNA. The annealing is most efficient if it involves the terminal nucleotide(s) of the acceptor RNA but can occur to sites at least 5 nucleotides from the 3′-end. These end-to-end jumps are similar to steps proposed to be part of the integration reaction of non-long terminal repeat retrotransposons and can explain chimeric integration products derived from multiple RNA templates.