The structure of BAcillus subtilis levansucrase at 3.8 A resolution

The tertiary structure of Bacillus subtilis levansucrase (EC 2.4.1.10) at 3.8 A resolution has been obtained from x-ray diffraction data by the method of multiple isomorphous replacements using three heavy atom derivatives. The crystals belong to space group P212121 with 1 molecule/crystallographic asymmetric unit. Unit cell dimensions are: a = 53.8 A, b = 67.7 A, and c = 125.6 A. Isomorphous heavy atom derivatives were prepared by soaking crystals in solutions containing K2Hg(CN)4, KAu(CN)2, and K2PtCl6. Precession photographs were taken of 29 reciprocal lattice planes of the native compound and three derivatives. The intensities were measured with an automatic integrating microdensitometer. The heavy atoms were located by means of two-dimensional difference Patterson and Fourier projections. The heavy atom parameters were refined by a least squares method. In the subsequent phases of calculatin, a final mean figure of merit equal to 0.74 was obtained for the independent reflectins. A 3.8 A electron density map based on phases from three heavy atom derivatives has been calculated. The electron density map of levansucrase suggests that the molecule is a very elongated ellipsoid with overall dimensions of 26 X 32 X 117 A. A model of levansucrae is also presented.