Scaling structure factor amplitudes in electron cryomicroscopy using X-raysolution scattering

The structure factors derived from electron cryomicroscopic images are modi fied by the contrast transfer function of the microscope's objective lens a nd other influences. The phases of the structure factors can be corrected i n a straightforward way when the positions of the contrast transfer functio n rings are determined. However, corrected amplitudes are also essential to yield an accurate distribution of mass in the reconstruction. The correct scale factors for the amplitudes are difficult to evaluate for data that ar e merged from many different micrographs. We opt to use X-ray solution scat tering intensity from a concentrated suspension of the specimen to correct the amplitudes of the spherically averaged structure factors. When this app roach is applied to the three-dimensional image data of ice-embedded acroso mal bundles, the core of a filament in a three-dimensional reconstruction o f the acrosomal bundle becomes denser and matches more closely the outer de nsity ascribed to scruin, (C) 1999 Academic Press.