Crystallization and preliminary X-ray analysis of ocr, the product of gene0.3 of bacteriophage T7

Ocr, the product of gene 0.3 of bacteriophage T7, prevents the action of re striction endonucleases of the host bacteria. The amino-acid sequence of oc r has less than 20% similarity to any protein of known three-dimensional st ructure. Ocr has been crystallized in a number of different crystal forms a nd X-ray data for the seleno-L-methionine-substituted form has been collect ed to a resolution of 1.8 Angstrom. The presence of caesium was found to be required for good crystal growth. Anomalous X-ray data was used to identif y possible positions for Se and Cs atoms in the unit cell.