SR2 elements, non-long terminal repeat retrotransposons of the RTE-1 lineage from the human blood fluke Schistosoma...

As in other eukaryotes, a substantial portion of the genome of the human blood flukes belonging to the genus Schistosoma appears to be composed of mobile genetic elements and other repetitive sequences. The constitutent elements and their relative organization are not well understood, although retroposons (the SM alpha elements) and a family of non-long terminal repeat (LTR) retrotransposons (the SR1 elements) have been reported from the genome of Schistosoma mansoni. Here, we report the presence of a second family of non-LTR retrotransposons from S. mansoni which we have termed the SR2 elements. SR2 elements are members of a recently described lineage of non-LTR retrotransposons typified by the RTE-1 non-LTR retrotransposon of Caenorhabditis elegans. We determined the sequence for approximately 3.9 kb of a consensus full-length SR2 element, which included a long 5' untranslated region (UTR), potential first and second open reading frames (ORFs) of 78 and 1,018 amino acid residues, respectively, a short 3' UTR, and an A-rich 3' terminus. SR2 elements were bound by target site duplications. The putative first and second ORFs did not overlap. The second ORF was homologous to retroviral pol and encoded an apurinic/apyrimidinic endonuclease and a reverse transcriptase. A number of extremely short SR2 elements of less than 0.5 kb, reminiscent of SINEs, were also characterized. These consisted solely of the 5' and 3' UTRs of full-length SR2 elements, having both ORFs deleted. Analysis indicated that these SINE-like SR2 elements were produced by replication of a SINE-like SR2 element, rather than by repeated deletions within larger SR2 elements.