Homing endonucleases are often grouped into four families based on distinct sequence motifs. One of these families is known as the His-Cys box homing endonucleases and contains two clusters of conserved histidine and cysteine residues over a central 100 amino acid region. At last count, 23 members of this family had been identified. The open reading frames (ORFs) of these proteins are contained within mobile group I introns found in nuclear rDNA genes of several protists. The nuclear location of these introns and ORFs is currently unique among the homing endonuclease families and poses an intriguing puzzle regarding their expression from non-coding rRNA transcripts.

The best-studied member of the His-Cys box homing endonucleases is I-Ppol from the myxomycete Physarum polycephalum. Following an introduction to all of the known members of the His-Cys box endonuclease family, much of the following chapter will outline the extensive characterization of I-Ppol structure and function. Although our understanding of how I-Ppol is expressed in cells is still not fully complete, the means by which I-Ppol specifically recognizes a single cleavage site in the host genome to mediate homing of its host intron is widely accepted. Details of DNA recognition and the catalytic mechanism of nucleolytic cleavage have been ascertained from both in vivo and in vitro activity assays as well as from extensive X-ray crystallo-graphic structural analyses of the enzyme bound to its DNA substrate.

E.A. Galburt (e-mail: [email protected])

Physical Biosciences, Lawrence Berkeley National Laboratory, 1 Cyclotron Road Mail Stop 20A-355, Berkeley, California 94720, USA

M.S. Jurica (e-mail: [email protected])

Molecular, Cell and Developmental Biology, Center for Molecular Biology of RNA, UC Santa Cruz, 1156 High Street, Santa Cruz, California 95064, USA

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

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