Evolutionarily recent groups of transposable elements in the human genome

Transposable elements (TEs) are fragments of DNA capable of self-reproduction in the genome of the host organism. They constitute ∼40–50% of mammalian genomes. We have identified two TE families that formed relatively recently in the course of human evolution. Members of the first family are present only in the human genome but are absent from other primate DNAs. This family represented by ∼80 members was formed by fusion of a portion of the CpG-island in the human MAST2 gene with the 3′-terminal fragment of the SVA retrotransposon. According to our estimates, this hybrid family, termed CpG-SVA , is significantly more active than the ancestor SVA family. The regulatory region of MAST2 allows copies of the new family to be transcribed in sperm precursor cells. The second family, called the family of chimera retrotranscripts, is older, but is still active today. Its representatives were formed by a rather unusual RNA recombination mechanism, which mediated the formation of fused DNA copies for diverse cellular transcripts. We showed that similar mechanisms operate in the genomes of other mammals and even fungi.