THE STRUCTURE OF I-CREI, A GROUP-I INTRON-ENCODED HOMING ENDONUCLEASE

The structure of I-Crel provides the first view of a protein encoded b y a gene within an intron. This endonuclease recognizes a long DNA sit e similar to 20 base pairs in length and facilitates the lateral trans fer of that intron. The protein exhibits a DNA-binding surface consist ing of four antiparallel beta-strands that form a 20 A wide groove whi ch is over 70 Angstrom long, The architecture of this fold is differen t from that of the TATA binding protein, TBP, which also contains an a ntiparallel beta-saddle. The conserved LACLIDADG motif, which is found in many mobile intron endonucleases, maturases and inteins, forms a n ovel helical interface and contributes essential residues to the activ e site.