Figure 1.

Structure of non-long terminal repeat (non-LTR) retrotransposons. The general structures of three model non-LTR elements are shown. Top: the Bombyx mori R2 element consists of a single open reading frame (ORF) containing reverse transcriptase (RT) activity and a restriction enzyme-like (REL) domain. Vertical lines indicate cysteine-histidine rich regions believed to encode nucleic acid binding domains. Horizontal lines are untranslated regions. Middle: the human L1 element contains two ORFs. ORF1 contains a leucine zipper (LZ) domain involved in protein-protein interactions and a C-terminal nucleic acid binding domain (BD). ORF2 has a N-terminal apurinic/apyrimidinic endonuclease (APE), a central RT domain, and a C-terminal cysteine-histidine rich domain. The element ends with a polyA tail. Typical insertions are flanked by target site duplications (bold arrows). Bottom: the Drosophila I factor element. ORF1 contains three C-terminal cysteine-histidine rich motifs resembling those of retroviral gag. ORF2 contains an APE endonuclease, central RT/RNaseH (RNH) domain, and C-terminal cysteine-histidine domain. In all structures contiguous gray boxes represent a single ORF.