CRYSTALLIZATION AND PRELIMINARY-X-RAY STUDIES OF I-PPOI - A NUCLEAR, INTRON-ENCODED HOMING ENDONUCLEASE FROM PHYSARUM-POLYCEPHALUM

The homing endonuclease I-PpoI is encoded by an optional third intron, Pp LSU 3, found in nuclear, extrachromosomal copies of the Physarum p olycephalum 26S rRNA gene. This endonuclease promotes the lateral tran sfer or ''homing'' of its encoding intron by recognizing and cleaving a partially symmetric, 15 bp homing site in 26S rDNA alleles that lack the Pp LSU 3 intron. Tile open reading frame encoding I-PpoI has been subcloned, and the endonuclease has been overproduced in E. coli. Pur ified recombinant I-PpoI has been co-crystallized with a 21 bp homing site DNA duplex. The crystals belong to space group P3(1)21, with unit cell dimensions a = b = 114 Angstrom, c = 89 Angstrom. The results of initial X-ray diffraction experiments indicate that the asymmetric un it contains an enzyme homodimer and one duplex DNA molecule, and that the unit cell has a specific volume of 3.4 Angstrom(3)/dalton. These e xperiments also provide strong evidence that I-PpoI contains several b ound zinc ions as part of its structure.