HUMAN L1 RETROTRANSPOSON ENCODES A CONSERVED ENDONUCLEASE REQUIRED FOR RETROTRANSPOSITION

Human L1 elements are highly abundant poly(A) (non-LTR) retrotransposo ns whose second open reading frame (ORF2) encodes a reverse transcript ase (RT). We have identified an endonuclease (EN) domain at the L1 ORF 2 N-terminus that is highly conserved among poly(A) retrotransposons a nd resembles the apurinic/apyrimidinic (AP) endonucleases. Purified L1 EN protein (L1 ENp) makes 5'-PO4, 3'-OH nicks in supercoiled plasmids , shows no preference for AP sites, and preferentially cleaves sequenc es resembling L1 in vivo target sequences. Mutations in conserved amin o acid residues of L1 EN abolish its nicking activity and eliminate L1 retrotransposition. We propose that L1 EN cleaves the target site for L1 insertion and primes reverse transcription.