Crystallization of the DpnM methylase from the DpnII restriction system of Streptococcus pneumoniae

Three proteins, two DNA methylases and an endonuclease, from the DpnII restriction system of Streptococcus pneumoniae recognize the DNA sequence 5' GATC 3' but have very different amino acid sequences, which make them interesting subjects for structural determination. A purification procedure was developed that conveniently yields milligram amounts of the DpnM methylase. The DpnM protein tends to precipitate at reduced ionic strength, and this property was exploited to yield well-formed bipyramidal crystals. By X-ray diffraction, the crystals of DpnM were found to be orthorhombic, with cell dimensions a = 56.9 A, b = 68.2 A, c = 84.5 A; systematic absences identify the space group as P2(1)2(1)2(1). Diffraction extends beyond 3 A, so the crystals may allow structural determination at atomic resolution.