Transduction of 3'-flanking sequences is common in L1 retrotransposition

Active LINE-1 (L1) elements possess the ability to transduce non-L1 DNA flanking their 3' ends to new genomic locations. Occasionally, the 3' end processing machinery may bypass the L1 polyadenylation signal and instead utilize a second downstream polyadenylation site. To determine the frequency of L1-mediated transduction in the human genome, we selected 66 previously uncharacterized L1 sequences from the GenBank database. Fifteen (23%) of these L1s had transposed flanking DNA with an average transduction length of 207 nucleotides. Since them are ~400,000 L1 elements, we estimate that insertion of transduced sequences alone may have enlarged the diploid human genome as much as 19 Mb or 0.6%. We also examined 24 full-length mouse L1s and found two long transduced sequences. Thus, L1 retrotransposition in vivo commonly transduces sequence flanking the 3' end of the element.