Identification of RNA binding motifs in the R2 retrotransposon-encoded reverse transcriptase

R2 non-LTR retrotransposons insert at a specific site in the 28S rRNA genes of many animal phyla. R2 elements encode a single polypeptide with reverse transcriptase, endonuclease and nucleic acid binding domains. Integration involves separate cleavage of the two DNA strands at the target site and utilization of the released 3' ends to prime DNA synthesis. Critical to this integration is the ability of the protein to specifically bind 3' and 5' regions of the R2 RNA. In this report, alanine mutations in two conserved motifs N-terminal to the reverse transcriptase domain were generated and shown to result in proteins that retained the ability to cleave the first strand of the DNA target, to reverse transcribe RNA from an annealed primer and to displace annealed RNA when using DNA as a template. However, the mutant proteins had greatly reduced ability to bind 3' and 5' RNA in mobility shift assays, use the DNA target to prime reverse transcription and conduct second-strand DNA cleavage. These motifs thus appear to participate in all activities of the R2 protein known to require specific RNA binding. The similarity of these R2 RNA binding motifs to those of telomerase and group II introns is discussed.