Circular retrotransposition products generated by a LINE retrotransposon.

Non-long terminal repeat (non-LTR) retrotransposons are highly abundant elements that are present in chromosomes throughout the eukaryotic domain of life. The long interspersed nuclear element (LINE-1) (L1) clade of non-LTR retrotransposons has been particularly successful in mammals, accounting for 30-40% of human genome sequence. The current model of LINE retrotransposition, target-primed reverse transcription, culminates in a chromosomally integrated end product. Using a budding yeast model of non-LTR retrotransposition, we show that in addition to producing these 'classical', chromosomally integrated products, a fungal L1 clade member (Zorro3) can generate abundant, RNA-derived episomal products. Genetic evidence suggests that these products are likely to be formed via a variation of target-primed reverse transcription. These episomal products are a previously unseen alternative fate of LINE retrotransposition, and may represent an unexpected source for de novo retrotransposition.