Figure 2.

Mechanism of LINE-1 target-primed reverse transcription (TPRT). Shown is a working model for canonical LINE-1 retrotransposition via TPRT. ORF2p EN initiates TPRT by nicking DNA at a relaxed target site (3′-AA/TTTT-5′) to liberate a 3′ OH at a short stretch of polyT, which binds the polyA tail of LINE-1 RNA to form a primer–template structure. ORF2p RT can then extend from the 3′ OH to generate LINE-1 cDNA using LINE-1 RNA as a template, resulting in a presumed LINE-1 RNA/cDNA hybrid intermediate. The subsequent steps of LINE-1 retrotransposition are poorly understood but may include the displacement or degradation of the LINE-1 RNA template, the cleavage of the second strand DNA, the synthesis of the second strand DNA, and the joining of the 5′ end of a double-stranded cDNA intermediate to genomic DNA. New copies of LINE-1 in the genome are characterized by signature features: They are enriched at EN target sequences, contain polyA tails in their 3′ end, and are flanked by short target site duplications (TSDs).