W0230

Low Resolution Molecular Replacement Using a Six-dimensional Search. Quan Hao, Qun Liu, MacCHESS, Wilson Synchrotron Lab, Cornell Univ., Ithaca, NY 14853, USA.

All the commonly used molecular replacement methods based on the Patterson function have difficulty in handling models derived from solution X-ray scattering or electron microscopy because of the low resolution of the models, usually worse than 10 Å. Even for a search model derived from the PDB (Protein Data Bank), conventional molecular replacement may not be able to yield correct solutions when the resolution of diffraction data is lower than 10 Å.

Here we report an exhaustive 6-dimensional search method for low resolution molecular replacement, using the molecular envelope derived from a PDB file and the diffraction data from 20 Å to 12 Å resolution. The program MPI FSEARCH has been implemented to locate an envelope in a crystallographic unit cell; it performs a simultaneous 6-dimensional search on orientation and translation to find the best match between experimental structure factors, F_obs, and calculated ones, F_c . The program can handle an envelope derived from various sources such as electron microscopy or small angle solution X-ray scattering, and no longer requires non-crystallographic symmetry information. Recent advances in parallel computer clusters and parallel libraries have enabled us to greatly expand the usability of MPI FSEARCH in the field of protein crystallography.