A density-modification procedure for improving maps from single-particle electron cryogenic microscopy (cryo-EM) is presented. The theoretical basis of the method is identical to that of maximum-likelihood density modification, previously used to improve maps from macromolecular X-ray crystallography. Key differences from applications in crystallography are that the errors in Fourier coefficients are largely in the phases in crystallography but in both phases and amplitudes in cryo-EM, and that half-maps with independent errors are available in cryo-EM. These differences lead to a distinct approach for combination of information from starting maps with information obtained in the density-modification process. The density-modification procedure was applied to a set of 104 datasets and improved map-model correlation and increased the visibility of details in many of the maps. The procedure requires two unmasked half-maps and a sequence file or other source of information on the volume of the macromolecule that has been imaged.

介绍了用于改进单粒子电子低温显微镜 (cryo-EM) 地图的密度修改程序。该方法的理论基础与最大似然密度修改的理论基础相同，以前用于改进大分子 X 射线晶体学的图谱。与晶体学应用的主要区别在于，傅里叶系数的误差主要存在于晶体学的相位中，但在低温 EM 中存在于相位和振幅中，并且具有独立误差的半图在低温 EM 中可用。这些差异导致了一种独特的方法，用于将来自起始地图的信息与在密度修改过程中获得的信息相结合。密度修改程序应用于一组 104 个数据集，改进了地图模型相关性并增加了许多地图中细节的可见性。该过程需要两个未屏蔽的半图和一个序列文件或其他有关已成像大分子体积的信息源。