Retrotransposition of the I factor, a non-long terminal repeat retrotransposon of Drosophila, generates tandem repeats at the 3′ end

Non-long terminal repeat (LTR) retrotransposons or LINEs transpose by reverse transcription of an RNA intermediate and are thought to use the 3′ hydroxyl of a chromosomal cleavage to initiate synthesis of the first strand of the cDNA. Many of them terminate in a poly(dA) sequence at the 3′ end of the coding strand although some, like the I factor of Drosophila melanogaster, have 3′ ends formed by repeats of the trinucleotide TAA. We report results showing that I factor transcripts end a few nucleotides downstream of the TAA repeats and that these extra nucleotides are not integrated into chromosomal DNA during retrotransposition. We also show that the TAA repeats are not required for transposition and that I elements containing mutations affecting the TAA sequences generate transposed copies ending with tandem repeats of various types. Our results suggest that during integration the 3′ end of the I factor RNA template can pair with nucleotides at the target site and that tandem duplications are generated by the reverse transcriptase of the I factor in a manner that is reminiscent of the activity of the reverse transcriptases of telomerases. Reverse transcriptases of other non-LTR retrotransposons may function in a similar way.