Crystallographic refinement of macromolecules having non-crystallographic symmetry

Some very large biological macromolecules, such as viruses, exhibit a high degree of non-crystallographic symmetry. A method is described to refine crystallographically such large structures using a combination of molecular averaging in real space, automatic real-space refitting and interactive refitting using computer graphics. The method has been successfully applied to a small plant virus, satellite tobacco necrosis virus, containing 11 700 amino acids in the crystallographic asymmetric unit. The starting model for the refinement was built with 3.7 Å phases. These have been refined and the resolution extended to 2.5 Å.