Reverse Transcriptase Encoded by a Human Transposable Element

L1 elements are highly repeated mammalian DNA sequences whose structure suggests dispersal by retrotransposition. A consensus L1 element encodes a protein with sequence similarity to known reverse transcriptases. The second open reading frame from the human L1 element L1.2A was expressed as a fusion protein targeted to Ty1 virus-like particles in Saccharomyces cerevisiae and shown to have reverse transcriptase activity. This activity was eliminated by a missense mutation in the highly conserved amino acid motif Y/F-X-D-D. Thus, L1 represents a potential source of the reverse transcriptase activity necessary for dispersion of the many classes of mammalian retroelements.