Tables for
Volume F
Crystallography of biological macromolecules
Edited by M. G. Rossmann and E. Arnold

International Tables for Crystallography (2006). Vol. F. ch. 21.1, p. 501   | 1 | 2 |

Section Symmetry

G. J. Kleywegta*

aDepartment of Cell and Molecular Biology, Uppsala University, Biomedical Centre, Box 596, SE-751 24 Uppsala, Sweden
Correspondence e-mail: Symmetry

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From the symmetry of the diffraction pattern, the point-group symmetry of the crystal lattice can usually be derived. It is important to merge the data in the point group with the highest possible symmetry (usually assessed using merging statistics) in order to minimize the chance of making an incorrect space-group assignment (Marsh, 1995[link], 1997[link]; Kleywegt et al., 1996[link]). Once the first data set has been processed, it is always useful to compute a self-rotation function. A non-origin peak of comparable strength to the origin peak will indicate that the true space group has higher symmetry. [Similarly, a self-Patterson function can be calculated at this stage to detect any purely translational NCS (Kleywegt & Read, 1997[link]).] Once the final model is available, a search for possibly missed higher symmetry can be carried out, e.g. using the method developed by Hooft et al. (1994[link]).

Sometimes crystallographic symmetry breaks down (pseudo-symmetry): an apparent higher symmetry at low resolution does not hold at higher resolution. In some cases, this is a consequence of the chemistry of the system studied (e.g. an asymmetric ligand bound by a symmetric protein dimer). In other cases, it may go undetected and complicate space-group determination and solution and refinement of the structure.

When it comes to space-group determination, many of the lessons learned by small-molecule crystallographers also apply to macromolecular crystallography (Marsh, 1995[link]; Watkin, 1996[link]).


First citation Hooft, R. W. W., Sander, C. & Vriend, G. (1994). Reconstruction of symmetry-related molecules from Protein Data Bank (PDB) files. J. Appl. Cryst. 27, 1006–1009.Google Scholar
First citation Kleywegt, G. J., Hoier, H. & Jones, T. A. (1996). A re-evaluation of the crystal structure of chloromuconate cycloisomerase. Acta Cryst. D52, 858–863.Google Scholar
First citation Kleywegt, G. J. & Read, R. J. (1997). Not your average density. Structure, 5, 1557–1569.Google Scholar
First citation Marsh, R. E. (1995). Some thoughts on choosing the correct space group. Acta Cryst. B51, 897–907.Google Scholar
First citation Marsh, R. E. (1997). The perils of Cc revisited. Acta Cryst. B53, 317–322.Google Scholar
First citation Watkin, D. (1996). Pseudo symmetry. In Proceedings of the CCP4 study weekend. Macromolecular refinement, edited by E. Dodson, M. Moore, A. Ralph & S. Bailey, pp. 171–184. Warrington: Daresbury Laboratory.Google Scholar

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