Preliminary crystallographic studies on glutamine synthetase from Mycobacterium tuberculosis

The etiologic agent of tuberculosis, Mycobacterium tuberculosis, has been shown to secrete the enzyme glutamine synthetase (TB-GS) which is apparently essential for infection. Four crystal forms of a recombinant TB-GS were grown. The one chosen for synchrotron X--ray data collection belongs to space group P212121 with unit-cell dimensions 208 x 258 x 274 A, yielding 2.4 A resolution data. A Matthews number of 2.89 A3 Da-1 is found, corresponding to 24 subunits of molecular mass 1300 kDa in the asymmetric unit. From earlier work, the structure of Salmonella typhimurium GS, which is 51% identical in sequence to TB-GS, is known to be dodecameric with 622 symmetry. Self-rotation calculations on the TB-GS X-ray data reveal only one set of sixfold and twofold axes of symmetry. A Patterson map calculated from the native X-ray data confirms that there are two dodecamers in the asymmetric unit, having both their sixfold and twofold axes parallel to one another.