Generation of highly site-specific DNA double-strand breaks in human cells by the homing endonucleases I-PpoI and I-CreI.

We have determined the ability of two well-characterized eukaryotic homing endonucleases, I-PpoI from the myxomycete Physarum polycephalum and I-CreI from the green alga Chlamydomonas reinhardtii, to generate site-specific DNA double-strand breaks in human cells. These 18-kDa proteins cleave highly conserved 15- or 24-bp rDNA homing sites in their respective hosts to generate homogeneous 4-base, 3' ends that initiate target intron transposition or "homing." We show that both endonucleases can be expressed in human cells and can generate site-specific DNA double-strand breaks in 28S rDNA and homing site plasmids. These endonuclease-induced breaks can be repaired in vivo, although break repair is mutagenic with the frequent generation of short deletions or insertions. I-PpoI and I-CreI should be useful for analyzing DNA double-strand break repair in human cells and rDNA.