MBE Advance Access published online on May 30, 2003
Molecular Biology and Evolution, doi:10.1093/molbev/msg141
Molecular Biology and Evolution © Society for Molecular Biology and Evolution 2003; all rights reserved
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1 Department of Biology, University of Rochester, Rochester, NY 14627-0211, USA
* To whom correspondence should be addressed. E-mail: eick{at}mail.rochester.edu.
A small (100 bp) region of the 28S rRNA gene has been shown to serve as the target site for the insertion of non-long terminal repeat (non-LTR) retrotransposons in both arthropods and nematodes. Here we characterize a lineage of non-LTR retrotransposons that inserts into this target site in the phylum Platyhelminthes. Dugesiid planaria contain elements, named R5, that insert 8 bp upstream of the target site used by arthropod R2 elements. The complete sequence of this element from Girardia tigrina revealed that it encoded two open reading frames (ORFs). The second ORF contained reverse transcriptase and restriction enzyme-like endonuclease domains similar to those found in R2 and R4, the element that inserts into the 28S genes of nematodes. The closest relative of R5, however, was the element, NeSL-1, which inserts into the spliced leader 1 exons of nematodes. The rRNA genes of dugesiid planaria are unusual in that they comprise two types of rDNA units that differ by 8-10% in nucleotide sequence of the 18S and 28S coding regions. Type II units are transcribed in adult tissues at levels that are less than 1% that of the type I units. R5 elements were only found inserted in the type II units where they presumably cause less harm to the host. A second unusual aspect of the dugesiid rRNA genes is that the target site for the R5 insertion is duplicated 300 bp upstream of the original insertion site. R5 elements were identified in both sites. These findings expand the distribution of non-LTR elements that are specialized for insertion into the 28S gene and suggest that still more elements exist in other eukaryotic taxa. Attempts to trace the phylogeny of R5 did not offer sufficient resolution to determine whether R2, R4 and R5 represent the same lineage or whether they represent independent specializations for the 28S gene. Key Words:
Retrotransposons, insertion specificity, rRNA gene, transcription, phylogenetic analysis
© 2003 Society for Molecular Biology and Evolution
Original Articles
R5 Retrotransposons Insert into a Family of Infrequently Transcribed 28S rRNA Genes of Planaria
2 Department of Biology, University of Rochester, Rochester, NY 14627-0211 USA
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