Retrotransposition of a mouse L1 element.

Long interspersed elements (LINEs) of the L1 family represent a major class of mammalian repetitive DNA and are present at copy numbers of between 10(4) and 10(5) elements per genome. Structural similarities between L1 elements and known retrotransposons have led to the suggestion that a subset of L1 elements may function as mobile genetic elements and have thus gained their prominent place in the mammalian genome. We describe a consensus mouse L1 element that was tagged with a heterologous intron and shown to transpose by way of an RNA intermediate when transfected into baby hamster kidney cells, formally establishing L1 elements as retrotransposons. When the putative reverse transcriptase-encoding region of this L1 element was deleted, the element still underwent retrotransposition in hamster cells, suggesting that reverse transcriptase activity can be supplied by an endogenous enzyme.