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== Publication Abstract from PubMed ==

The homing endonuclease I-Ssp6803I causes the insertion of a group I intron into a bacterial tRNA gene-the only example of an invasive mobile intron within a bacterial genome. Using a computational fold prediction, mutagenic screen and crystal structure determination, we demonstrate that this protein is a tetrameric PD-(D/E)-XK endonuclease - a fold normally used to protect a bacterial genome from invading DNA through the action of restriction endonucleases. I-Ssp6803I uses its tetrameric assembly to promote recognition of a single long target site, whereas restriction endonuclease tetramers facilitate cooperative binding and cleavage of two short sites. The limited use of the PD-(D/E)-XK nucleases by mobile introns stands in contrast to their frequent use of LAGLIDADG and HNH endonucleases - which in turn, are rarely incorporated into restriction/modification systems.

The restriction fold turns to the dark side: a bacterial homing endonuclease with a PD-(D/E)-XK motif.,Zhao L, Bonocora RP, Shub DA, Stoddard BL EMBO J. 2007 May 2;26(9):2432-42. Epub 2007 Apr 5. PMID:17410205<ref>PMID:17410205</ref>

From MEDLINE&reg;/PubMed&reg;, a database of the U.S. National Library of Medicine.<br>

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== References ==

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