RETROTRANSPOSON R1BM ENDONUCLEASE CLEAVES THE TARGET SEQUENCE

The R1Bm element, found in the silkworm Bombyx mori, is a member of a group of widely distributed retrotransposons that lack long terminal r epeats. Some of these elements are highly sequence-specific and others , like the human L1 sequence, are less so. The majority of R1Bm elemen ts are associated with ribosomal DNA (rDNA). R1Bm inserts into 28S rDN A at a specific sequence; after insertion it is flanked by a specific 14-bp target site duplication of the 28S rDNA. The basis for this sequ ence specificity is unknown. We show that R1Bm encodes an enzyme relat ed to the endonuclease found in the human L1 retrotransposon and also to the apurinic/apyrimidinic endonucleases. We expressed and purified the enzyme from bacteria and showed that it cleaves in vitro precisely at the positions in rDNA corresponding to the boundaries of the 14-bp target site duplication. We conclude that the function of the retrotr ansposon endonucleases is to define and cleave target site DNA.