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The extracellular thermostable xylanase (XT-6) produced by the thermophilic bacterium Bacillus stearothermophilus T-6 was shown to bleach pulp optimally at pH 9 and 338 K, and was successfully used in a large-scale biobleaching mill trial. The xylanase gene was cloned and sequenced. The mature enzyme consists of 379 amino acids with a calculated molecular weight of 43 808 and pI of 9.0. Crystallographic studies of XT-6 were initiated to study the mechanism of catalysis as well as to provide a structural basis for rational introduction of enhanced thermostability by site-specific mutagenesis. This report describes the crystallization and preliminary crystallographic characterization of the native XT-6 enzyme. The most suitable crystals were obtained by the vapor-diffusion method using ammonium sulfate and 2-methyl-2,4-pentanediol as an organic additive. The crystals belong to a primitive trigonal crystal system (space group P31 or P32) with room-temperature cell dimensions of a = b = 114.9 and c = 122.6 Å. At 103 K the volume of the unit cell decreased significantly with observed dimensions of a = b = 112.2 and c = 122.9 Å. These crystals are mechanically strong and diffract X-rays to better than 2.2 Å resolution. The crystals exhibit considerable radiation damage at room temperature even at relatively short exposures to X-rays. A full 2.3 Å resolution diffraction data set (99.8% completeness) has recently been collected on flash-frozen crystals at 103 K using synchrotron radiation. Two derivatives of XT-6 were recently prepared. In the first derivative, a unique Cys residue replaced Glu265, the putative nucleophile in the active site. The second derivative was selenomethionyl xylanase which was produced biosynthetically. These derivatives have been crystallized and the resulting crystals were shown to be isomorphous to the native crystals and diffract X-rays to comparable resolutions.
