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homing endonuclease I-PpoI Homing endonuclease I-PpoI is a member of the "His-Cys box" homing endonuclease family. In the slime mold Physarum polycephalum, I-PpoI is encoded by an open reading frame in the third intron of the nuclear, extrachromosomal 26 S rRNA gene. The "His-Cys box" sequence motifs are responsible for forming two structural zinc-binding sites per monomer and the enzyme is dependent on divalent cations (usually magnesium) for activity. I-PpoI is a homodimer that cleaves a partially symmetric 15 bp homing site generating four base, 3' cohesive overhangs of sequence 5'-TTAA-3'. Homing sites may vary in sequence at many of their individual nucleotide positions while still being recognized and cleaved. While I-PpoI tolerates base-pair substitutions at several positions within the homing site, it exhibits a strong preference for AT base-pairs in the central four positions of the homing site. I-PpoI severely bends its DNA target, resulting in significant deformations of the minor and major groove near the scissile phosphate groups, and only a smaller change in protein conformation. |
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