Fig. 1
The biology of a LINE-1 retrotransposon. The structure of a human L1 is shown. TSD: target site duplication; UTR: untranslated region; EN: endonuclease; RT: reverse transcriptase; C: carboxy-terminal segment; An: polyadenylation signal and tail. The LINE-1 replication cycle involves transcription and export of its RNA to the cytoplasm, which is translated and assembled in a ribonucleoprotein particle (RNP) together with L1 ORF1p and ORF2p. There is a strong cis-preference for L1 ORF1 and ORF2 proteins to bind their own encoding RNA in a retrotransposition-competent RNP. ORF1p binds L1 RNA as a trimer, however, it is unclear if it remains bound at the time of import of the RNP into the nucleus (denoted by ?) ) [20,21,22]. Reverse transcription of LINE-1 RNA to generate complementary (cDNA) occurs at the site of chromosomal insertion by TPRT [23]. L1s frequently become 5′-truncated when inserted in the genome