Unique Mammalian tRNA-Derived Repetitive Elements in Dermopterans: The t-SINE Family and Its Retrotransposition Through Multiple Sources

Short interspersed nuclear elements (SINEs) are dispersed repetitive DNA sequences that are major components of all mammalian genomes. They have been described in almost all lineages of Euarchontoglires (rodents, rabbits, primates, flying lemurs, and tree shrews), except in flying lemurs. Most SINE family members are composed of three distinct regions: a 5' tRNA-related region, a tRNA-unrelated region, and a short tandem repeat at the 3' end that is AT-rich. The newly discovered SINE family in Cynocephalus deviates from this common structure. All 30 SINE loci analyzed in this family lack a tRNA-unrelated region and are composed exclusively of tRNA-related elements. Therefore, this novel SINE structure, described for the first time in mammalian genomes, was designated as t-SINE. The t-SINE family exhibits a high copy number and is specific to flying lemurs. Three major t-SINE subfamilies could be distinguished on the basis of characteristic nucleotides, deletions, insertions, and duplications. These sequence-specific characteristics within subfamilies and sub-subfamilies reveal that they are derived copies of distinct progenitors. We present evolutionary relationships between subfamilies and compare relationships between the subfamilies and the isoleucine tRNA gene. t-SINE amplification occurred through multiple sources and is supposedly mobilized via the L1-encoded reverse transcriptase-dependent retrotranspositional mechanism in trans.