Group II intron homing endonucleases are ribonucleoproteins (RNPs) consisting of a catalytically active intron RNA and an intron-encoded protein (IEP), with reverse transcriptase (RT) and/or DNA endonuclease (En) activity. The RNP is formed when the IEP binds to the intron in unspliced RNA and promotes its splicing by stabilizing the catalytically active RNA structure. Afterwards, the IEP remains tightly bound to the excised intron RNA to constitute the homing endonuclease. The homing endonuclease promotes intron mobility by a remarkable mechanism in which the intron RNA reverse splices directly into a target DNA and is then reverse-transcribed by the IEP. Importantly, the target site for intron insertion is determined mainly by base pairing between short sequence elements in the intron RNA and target DNA, making it straightforward to change the target specificity of the homing endonuclease simply by modifying the intron RNA. This feature combined with their very high specificity and insertion frequencies have made it possible to develop mobile group II introns into gene targeting vectors, called "targetrons", with programmable target specificity.

Here, we focus on the structure, function, and interaction of the IEP and intron RNA in the homing endonuclease. We discuss in detail how group II

A.M. Lambowitz (e-mail: [email protected]), G. Mohr

Institute for Cellular and Molecular Biology, Department of Chemistry and Biochemistry, and Section of Molecular Genetics and Microbiology, School of Biological Sciences, University of Texas at Austin, Austin, Texas 78712 USA

S. Zimmerly

University of Calgary, Department of Biological Sciences, 2500 University Drive NW, Calgary, AB,T2N 1N4, Canada

Nucleic Acids and Molecular Biology, Vol. 16 Marlene Belfort et al. (Eds.) Homing Endonucleases and Inteins © Springer-Verlag Berlin Heidelberg 2005

intron homing endonucleases recognize DNA target sites and how they can be used for targeted gene disruption and site-specific DNA insertion in bacteria, or potentially, in eukaryotes.

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