The design and operation of a focusing camera for high-resolution macromolecular crystallography with synchrotron radiation (SR) are described. The performance of this service-oriented instrument is evaluated on the basis of five years of use. Standard procedures for data collection, data processing and data reduction have been modified to take unusual features of the SR source into account; the effect of polarization is thoroughly discussed.

A survey of 129 protein crystal structures with more than one molecule per asymmetric unit shows that local (non-crystallographic) symmetry axes are not randomly oriented. When compared to the crystal cell edges, face diagonals, body diagonal and reciprocal cell edges, 65% of the local symmetry axes are found to be parallel to one of the reference directions to within 15 degrees; another 18% are orthogonal to within 3 degrees; only 17% are in general orientations. In monoclinic, trigonal and hexagonal crystals, a majority of the local symmetry axes are orthogonal to the unique axis, while preferred orientations are parallel to the cell edges in orthorhombic crystals.

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TAOS is a fully automatic system for protein crystallization. Based on a central five-axis robot, it performs vapour-diffusion experiments in commercially available plates with three types of drop geometry, i.e. hanging, sitting or sandwich drops. It can handle ten crystallization plates (up to 240 tests) and prepare precipitant solutions from up to ten different components with no operator intervention. Software help in preparing crystallization experiments and full archiving are provided. TAOS has been thoroughly tested for accuracy and reproducibility.