COMBINING CRYO-EM AND X-RAY DIFFRACTION STRUCTURE RESULTS FOR SPHERICAL VIRUSES. Michael G. Rossmann, Department of Biological Sciences, Purdue University, West Lafayette, IN 47907-1392

Cryo-EM structures of spherical viruses usually have a limiting resolution of around 22 Å. X-ray diffraction data are usually somewhat incomplete inside 25 Å resolution, on account of being obscured by the beam stop. Thus, phasing of the X-ray data, using the EM structure as a model, frequently lacks sufficient overlap to permit successful phase extension using the non-crystallographic symmetry. These problems can be alleviated both by carefully matching the radial scale of the EM image to the X-ray data and by assigning a crude atomic structure to the density. This can be based either on a rough structural interpretation or on suitably randomly placed atoms within the boundary of the EM density.

Interpretation of the EM density with reasonable models based on structural components of the virus requires checking for the uniqueness of the proposed fit. A variety of biological information can be used to support the results. Ross River virus (an enveloped +RNA virus) and [[phi]]X174 (a ssDNA phage) assembly intermediates will be used as examples.