Explicit correction of severely non-uniform distributions of cryo-EM views

The quality of three-dimensional macromolecular image reconstruction by cryo electron microscopy (cryo-EM) strongly depends on the number and the quality of the respective two-dimensional projections and on their angular distribution in space. Distributions with one or a few strongly preferred particle orientations may result in maps that are deformed in certain directions. A simple removal of overrepresented views may improve the quality of the reconstructed maps when the level of noise in the two-dimensional (2D) projections is low and the data-set size can afford this removal, but is counterproductive otherwise. Complementarily, giving an increased weight to underrepresented views, or taking multiple copies of them during the reconstruction, may improve the results, naturally, depending on how non-uniform the view distribution is. This work describes the results of three-dimensional (3D) reconstructions using an explicit correction of the number of overrepresented and underrepresented projections for non-uniformly distributed sets. Such correction can be considered as a potential preprocessing, fast and simple, during 3D reconstruction in the image-processing and cryo-EM structure-determination workflow.