Published online 7 April 2005
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Site-specific reverse splicing of a HEG-containing group I intron in ribosomal RNA
1Department of Molecular Biotechnology, Institute of Medical Biology, University of Tromsø N-9037 Tromsø, Norway 2Faculty of Fisheries and Natural Sciences, Bodø Regional University N-8049 Bodø, Norway
*To whom correspondence should be addressed. Tel: +47 77 64 53 67; Fax: +47 77 64 53 50; Email: Steinar.Johansen{at}fagmed.uit.no
Received January 18, 2005. Revised March 18, 2005. Accepted March 18, 2005.
The wide, but scattered distribution of group I introns in nature is a result of two processes; the vertical inheritance of introns with or without losses, and the occasional transfer of introns across species barriers. Reversal of the group I intron self-splicing reaction, termed reverse splicing, coupled with reverse transcription and genomic integration potentially mediate an RNA-based intron mobility pathway. Compared to the well characterized endonuclease-mediated intron homing, reverse splicing is less specific and represents a likely explanation for many intron transpositions into new genomic sites. However, the frequency and general role of an RNA-based mobility pathway in the spread of natural group I introns is still unclear. We have used the twin-ribozyme intron (Dir.S956-1) from the myxomycete Didymium iridis to test how a mobile group I intron containing a homing endonuclease gene (HEG) selects between potential insertion sites in the small subunit (SSU) rRNA in vitro, in Escherichia coli and in yeast. Surprisingly, the results show a site-specific RNA-based targeting of Dir.S956-1 into its natural (S956) SSU rRNA site. Our results suggest that reverse splicing, in addition to the established endonuclease-mediated homing mechanism, potentially accounts for group I intron spread into the homologous sites of different strains and species.
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