Downstream 28S Gene Sequences on the RNA Template Affect the Choice of Primer and the Accuracy of Initiation by the R2 Reverse Transcriptase

Abstract

R2 non-long terminal repeat retrotransposable elements insert at a unique site in the 28S rRNA genes of insects. The protein encoded by the single open reading frame of R2 is capable of conducting the initial steps of its integration in vitro. The protein nicks the noncoding strand of the 28S target DNA (the strand which serves as a template for RNA synthesis) and uses the 3′ hydroxyl group exposed by this nick to prime reverse transcription of the R2 RNA template. This target-primed reverse transcription (TPRT) reaction requires that the RNA template contains the 250-nucleotide 3′ untranslated region of the R2 element. If this RNA template ends at the precise 3′ end of the R2 element, then extra nucleotides, which we refer to as nontemplated nucleotides, are added to the target before cDNA synthesis. The presence of downstream 28S gene sequences on the RNA template reduces the total efficiency but eliminates these nontemplated additions, resulting in nearly 90% of all TPRT products reproducing the 3′ junctions seen in vivo. Templates with 5 to 10 nucleotides of the 28S sequence are used most efficiently in this in vitro TPRT reaction. The requirement for downstream 28S rRNA sequences probably explains why the R2 elements of most insects differ from the majority of non-long terminal repeat retrotransposons in that they do not contain an A-rich repeat at their 3′ junction with the target DNA. The presence of downstream sequences on these in vitro R2 templates also revealed that the R2 reverse transcriptase can prime cDNA synthesis by using the 3′ end of another RNA molecule. This RNA-primed cDNA synthesis is not based on sequence complementarity between the RNA primer and the R2 template. The ability to use the 3′ end of a noncomplementary RNA molecule has also been seen with the reverse transcriptase of the mitochondrial Mauriceville plasmid of Neurospora crassa.