4-Å Projection Map of Bacteriophage T4 DNA Helix-Destabilizing Protein (gp32^*I) Crystal by 400-kV Electron Cryomicroscopy

Ice-embedded crystals of bacteriophage T4 DNA helix-destabilizing protein gp32^*I were imaged by computer-controlled spot-scanning on a 400-kV electron cryomicroscope. gp32^*I crystals generally have different steps of thickness within a crystal; each step can have different symmetry. Multivariate statistical analysis enabled us to unambiguously select spot-scan images that consist entirely of one motif which were processed subsequently by crystallographic Fourier-averaging techniques. The computed phases of the resulting reflections were evaluated for symmetry in projection, and some of those images were correlated with independent thickness measurements of freeze-dried samples of the same crystals. The structure factors with pgg symmetry from nine spot-scan images were merged, and the mean figure of merit of merged phases was better than 0.9 for data at resolution up to 4 Å. A projection map was generated and showed multiple density peaks that corresponded to the high-resolution features of gp32^*I.