L1 retrotransposons comprise 17% of the human genome. Although most L1s are inactive, some elements remain capable of retrotransposition. L1 elements have a long evolutionary history dating to the beginnings of eukaryotic existence. Although many aspects of their retrotransposition mechanism remain poorly understood, they likely integrate into genomic DNA by a process called target primed reverse transcription. L1s have shaped mammalian genomes through a number of mechanisms. First, they have greatly expanded the genome both by their own retrotransposition and by providing the machinery necessary for the retrotransposition of other mobile elements, such as Alus. Second, they have shuffled non-L1 sequence throughout the genome by a process termed transduction. Third, they have affected gene expression by a number of mechanisms. For instance, they occasionally insert into genes and cause disease both in humans and in mice. L1 elements have proven useful as phylogenetic markers and may find other practical applications in gene discovery following insertional mutagenesis in mice and in the delivery of therapeutic genes.
(Ostertag, E.M., Kazazian, Jr., H.H. Biology of Mammalian L1 Retrotransposons. Annu. Rev. Genet. 35:501-38, 2001.)
Active L1 (LINE-1) 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 nts. Since there are about 400,000 L1 elements, we estimate that insertion of transduced sequences alone may have enlarged the diploid human genome by 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.
(Goodier, J.L., Ostertag, E.M., Kazazian, Jr., H.H. Transduction of 3’ flanking sequences is common in L1 retrotransposition. Human Molecular Genetics. 9(4):653-7, 2000.)