Cross-genome screening of novel sequence-specific non-LTR retrotransposons: various multicopy RNA genes and microsatellites are selected as targets

Although most LINEs (long interspersed nuclear elements), which are autonomous non-long-terminal-repeat retrotransposons, are inserted throughout the host genome, three groups of LINEs, the early-branched group, the Tx group, and the R1 clade, are inserted into specific sites within the target sequence. We previously characterized the sequence specificity of the R1 clade elements. In this study, we screened the other two groups of sequence-specific LINEs from public DNA databases, reconstructed elements from fragmented sequences, identified their target sequences, and analyzed them phylogenetically. We characterized 13 elements in the early-branched group and 13 in the Tx group. In the early-branched group, we identified R2 elements from sea squirts and zebrafish in this study, although R2 has not been characterized outside the arthropod group to date. This is the first evidence of cross-phylum distribution of sequence-specific LINEs. The Dong element also occurs across phyla, among arthropods and mollusks. In the Tx group, we characterized five novel sequence-specific families: Kibi for TC repeats, Koshi for TTC repeats, Keno for the U2 snRNA gene, Dewa for the tRNA tandem arrays, and Mutsu for the 5S rRNA gene. Keno and Mutsu insert into the highly conserved region within small RNA genes and destroy the targets. Several copies of Dewa insert different positions of tRNA tandem array, which indicates a certain "site specifier" other than sequence-specific endonuclease. In all three groups, LINEs specific for the rRNA genes or microsatellites can occur as multiple families in one organism. This indicates that the copy number of a target sequence is the primary factor to restrict the variety of sequence specificity of LINEs.