Characterization of novel Alu- and tRNA-related SINEs from the tree shrew and evolutionary implications of their origins

We characterized two novel 7SL RNA-derived short interspersed nuclear element (SINE) families (Tu types I and 11) and a novel tRNA-derived SINE family (Tu type 111) from the tree shrew (Tupaia belangeri). Tu type I contains a monomer unit of a 7SL RNA-derived Alu-like sequence and a tRNA-derived region that includes internal RNA polymerase III promoters. Tu type 11 has a similar hybrid structure, although the monomer unit of the 7SL RNA-derived sequence is replaced by a dimer. Along with the primate Alu, the galago Alu type 11, and the rodent 131, these two families represent the fourth and fifth 7SL RNA-derived SINE families to be identified. Furthermore, comparison of the Alu domains of Tu types I and 11 with those of other 7SL RNA-derived SINEs reveals that the nucleotides responsible for stabilization of the Alu domain have been conserved during evolution, providing the possibility that these conserved nucleotides play an indispensable role in retropositional activity. Evolutionary relationships among these 7SL RNA-derived SINE families, as well as phylogenetic relationships of their host species, are discussed.