An actively retrotransposing, novel subfamily of mouse L1 elements.

Retrotransposition of LINEs and other retroelements increases repetition in mammalian genomes and can cause deleterious mutations. Recent insertions of two full-length L1s, L1spa and L1Orl, caused the disease phenotypes of the spastic and Orleans reeler mice respectively. Here we show that these two recently retrotransposed L1s are nearly identical in sequence, have two open reading frames and belong to a novel subfamily related to the ancient F subfamily. We have named this new subfamily TF (for transposable) and show that many full-length members of this family are present in the mouse genome. The TF 5' untranslated region has promoter activity, and TF-type RNA is abundant in cytoplasmic ribonucleoprotein particles, which are likely intermediates in retrotransposition. Both L1spa and L1Orl have reverse transcriptase activity in a yeast-based assay and retrotranspose at high frequency in cultured cells. Together, our data indicate that the TF subfamily of L1s contains a major class of mobile elements that is expanding in the mouse genome.