LAUSR

Non–long terminal repeat (non-LTR) retrotransposons, or long interspersed nuclear elements (LINEs), are an abundant class of eukaryotic transposons that insert into genomes by target-primed reverse transcription (TPRT). During TPRT, a target DNA sequence is nicked and primes reverse transcription of the retrotransposon RNA. Here, we report the cryo–electron microscopy structure of the Bombyx mori R2 non-LTR retrotransposon initiating TPRT at its ribosomal DNA target. The target DNA sequence is unwound at the insertion site and recognized by an upstream motif. An extension of the reverse transcriptase (RT) domain recognizes the retrotransposon RNA and guides the 3′ end into the RT active site to template reverse transcription. We used Cas9 to retarget R2 in vitro to non-native sequences, suggesting future use as a reprogrammable RNA-based gene-insertion tool. Description A retrotransposon zooms in on its target Much of the human genome consists of repetitive sequences called transposons that can copy and paste themselves into new DNA locations. The most common, and only demonstrably active, type of transposon in humans is the non–long terminal repeat retrotransposon, which uses an RNA intermediate that is converted into DNA directly at the site of a new insertion. Wilkinson et al. used cryo–electron microscopy to show what determines the sequence that is inserted and where it is inserted. They also show that the specificity can be altered, suggesting that these retrotransposons could be engineered as gene insertion tools. —DJ A retrotransposon structure elucidates the principles of target DNA selection and self-RNA recognition.