Crystallographic studies of Escherichia coli citrate synthase.

The citrate synthase from Escherichia coli B has been crystallized in a cubic space group with a unit cell spacing of 220 A. X-ray diffraction, electron microscopy, symmetry considerations, and low resolution projection Patterson syntheses are consistent with a model proposed in which 24 tetrameric molecules of Mr = 188,000 +/- 12,000 occupy the unit cell. The space group is apparently P23, although at low resolution the observed systematic absences in reflections are consistent with the space group P43n, a space group not allowed for asymmetric molecules. Estimates of VM suggest that in the true space group, P23, two tetrameric molecules occupy the asymmetric unit.