The Ancient Mariner Sails Again: Transposition of the Human Hsmar1 Element by a Reconstructed Transposase and Activities of the SETMAR Protein on Transposon Ends

Hsmar1, one of the two subfamilies of mariner transposons in humans, is an ancient element that entered the primate genome lineage approximately 50 million years ago. Although Hsmar1 elements are inactive due to mutational damage, one particular copy of the transposase gene has apparently been under selection. This transposase coding region is part of the SETMAR gene, in which a histone methylatransferase SET domain is fused to an Hsmar1 transposase domain. A phylogenetic approach was taken to reconstruct the ancestral Hsmar1 transposase gene that we named Hsmar1-Ra. The Hsmar1-Ra transposase efficiently mobilizes Hsmar1 transposons by a cut-and-paste mechanism in human cells and zebrafish embryos. Hsmar1-Ra can also mobilize short inverted-repeat transposable elements (MITEs) related to Hsmar1 (MiHsmar1), thereby establishing a functional relationship between an Hsmar1 transposase source and these MITEs. MiHsmar1 excision is two orders-of-magnitude more efficient than that of long elements, thus providing an explanation for their high copy number. We show that the SETMAR protein binds, and introduces single-strand nicks into Hsmar1 inverted repeat sequences in vitro. Pathway choice for DNA break repair was found to be characteristically different in response to transposon cleavage mediated by Hsmar1-Ra and SETMAR in vivo. Whereas nonhomologous end-joining plays a dominant role in repairing excision sites generated by the Hsmar1-Ra transposase, DNA repair following cleavage by SETMAR predominantly follows a homology-dependent pathway. The novel transposon system can be a useful tool for genome manipulations in vertebrates, and for investigations into the transpositional dynamics and contribution of these elements to primate genome evolution