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MBE Advance Access published online on July 26, 2006
Molecular Biology and Evolution, doi:10.1093/molbev/msl067
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© The Author 2006. Published by Oxford University Press on behalf of the Society for Molecular Biology and Evolution. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org
Accepted July 18, 2006

Research Article

Identification of rDNA-specific non-LTR Retrotransposons in Cnidaria

Kenji K. Kojima 1 *, Kei-ichi Kuma 1, Hiroyuki Toh 2, and Haruhiko Fujiwara 3

1 Institute for Chemical Research, Kyoto University
2 Medical Institute of Bioregulation, Kyushu University
3 Department of Integrated Biosciences, Graduate School of Frontier Sciences, University of Tokyo

* To whom correspondence should be addressed.
Kenji K. Kojima, E-mail: kojimakk{at}kuicr.kyoto-u.ac.jp


  Abstract

Ribosomal RNA genes are abundant repetitive sequences in most eukaryotes. Ribosomal DNA (rDNA) contains many insertions derived from mobile elements including non-long terminal repeat (non-LTR) retrotransposons. R2 is the well-characterized 28S rDNA-specific non-LTR retrotransposon family that is distributed over at least four bilaterian phyla. R2 is a large family sharing the same insertion specificity, and classified into four clades (R2-A, -B, -C, and -D) based on the N-terminal domain structure and the phylogeny. There is no observation of horizontal transfer of R2; therefore, the origin of R2 dates back to before the split between protostomes and deuterostomes. Here, we in silico identified one R2 element from the sea anemone Nematostella vectensis, and two R2-like retrotransposons from the hydrozoan Hydra magnipapillata. R2 from N. vectensis was inserted into the 28S rDNA like other R2, but the R2-like elements from H. magnipapillata were inserted into the specific sequence in the highly conserved region of the 18S rDNA. We designated the Hydra R2-like elements R8. R8 is inserted at 37-bp upstream from R7, another 18S rDNA-specific retrotransposon family. There is no obvious sequence similarity between targets of R2 and R8, probably because they recognize long DNA sequence. Domain structure and phylogeny indicate that R2 from N. vectensis is the member of the R2-D clade, and R8 from H. magnipapillata belongs to the R2-A clade despite its different sequence specificity. These results suggest that R2 had been generated before the split between cnidarians and bilaterians, and that R8 is a retrotransposon family that changed its target from the 28S rDNA to the 18S rDNA.

Keywords: non-LTR retrotransposon; rDNA; sequence specificity; R2; R8; Cnidaria.
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