Two-fold symmetry of crystalline DNA-EcoRI endonuclease recognition complexes

Recognition complexes between EcoRI endonuclease and either of two synthetic oligonucleotides (sequences CGCGAATTCGCG and TCGCGAATTCGCG) crystallize in Space Group P321 with unit cell parameters a = 128 and c = 47 A and a = 118.4 and c = 49.7 A, respectively. Native diffraction data to 3 A resolution have been collected from the form containing the tridecameric sequence. Electrophoretic analyses of dissolved crystals demonstrate that this form contains DNA and protein in a ratio of one double helix per enzyme dimer. The most likely asymmetric unit contents are one 31,000 dalton enzyme subunit and one strand of DNA, yielding VM values of 3.1 A3/dal and 2.8 A3/dal for the forms containing dodecameric and tridecameric DNA, respectively. This implies that the DNA-protein complex possesses two-fold rotational symmetry, which has been incorporated in the crystalline lattice.