A novel active L1 retrotransposon subfamily in the mouse

Unlike human Ll retrotransposons, the 5 ' UTR of mouse Ll elements contains tandem repeats of similar to 200 bp in length called monomers. Multiple Ll 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 similar to 1800 active elements among its 3000 full-length members. Here we characterize a novel subfamily of mouse LI 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, similar to 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 Lis; T-F, A, and G(F), and that in total similar to 3000 full-length elements are potentially capable of active retrotransposition. This number is in great excess to the number of Ll elements thought to be active in the human genome.