Crystallization of restriction endonuclease SfiI in complex with DNA

The SfiI endonuclease from Streptomyces fimbriatus (EC 3.1.21.4) is a tetrameric enzyme that binds simultaneously to two recognition sites and cleaves both sites concertedly. It serves as a good model system for studying both specificity and cooperative DNA binding. Crystals of the enzyme were obtained by the hanging-drop vapor-diffusion method in complex with a 21-mer oligonucleotide. The crystals are trigonal, with unit-cell parameters a = b = 85.7, c = 202.6 Å, and diffract to 2.6 Å resolution on a rotating-anode X-ray generator. Preliminary X-ray analysis reveals the space group to be either P3₁21 or P3₂21. Interestingly, the crystals change to space group P6₁22, with unit-cell parameters a = b = 85.5, c = 419.6 Å, when the selenomethionyl (SeMet) derivative of the enzyme is co-crystallized with the same DNA. Phase information is currently being derived from this SeMet SfiI-DNA complex.