A novel active L1 retrotransposon subfamily in the mouse

Unlike human L1 retrotransposons, the 5′ UTR of mouse L1 elements contains tandem repeats of ∼200 bp in length called monomers. Multiple L1 subfamilies exist in the mouse which are distinguished by their monomer sequences. We previously described a young subfamily, called the T_F subfamily, which contains ∼1800 active elements among its 3000 full-length members. Here we characterize a novel subfamily of mouse L1 elements, G_F, which has unique monomer sequence and unusual patterns of monomer organization. A majority of these G_F elements also have a unique length polymorphism in ORF1. Polymorphism analysis of G_F elements in various mouse subspecies and laboratory strains revealed that, like T_F, the G_F subfamily is young and expanding. About 1500 full-length G_F elements exist in the diploid mouse genome and, based on the results of a cell culture assay, ∼400 G_F elements are potentially capable of retrotransposition. We also tested 14 A-type subfamily elements in the assay and estimate that about 900 active A elements may be present in the mouse genome. Thus, it is now known that there are three large active subfamilies of mouse L1s; T_F, A, and G_F, and that in total ∼3000 full-length elements are potentially capable of active retrotransposition. This number is in great excess to the number of L1 elements thought to be active in the human genome.