AUTOMATED PROCEDURE FOR CONSTRUCTING AND SELECTING OPTIMIZED INITIAL MODELS

这个子项目是许多利用资源的研究子项目之一 由NIH/NCRR资助的中心拨款提供。子项目的主要支持 而子项目的主要调查员可能是由其他来源提供的， 包括其它NIH来源。 列出的子项目总成本可能 代表子项目使用的中心基础设施的估计数量， 而不是由NCRR赠款提供给子项目或子项目工作人员的直接资金。 在单粒子电子显微镜（EM）中，电子密度图的重建 从根本上取决于欧拉角的分配，描述的方向， 对应的2D投影图像。一个正确的分配将使正确的反投影 在3D空间中的图像，以创建适当的体积，而不正确的分配 会产生不同程度的像差因此，挑战在于确定真正的欧拉 所有投影的角度。这一问题可以通过直接倾斜试样来解决 在显微镜内或通过2D内的角度关系的识别 图像本身（全面的评论，见（Frank，2006 a））。当2D图像 在EM网格上表示均匀大分子复合物的至少几个视图 并且可以通过定义明确的对比度传递函数（CTF）值来描述，它们包含所有 欧拉角分配的必要信息，而不需要额外的 宏观倾斜根据中心截面定理， 投影图像表示通过3D傅立叶变换的切片（中央部分）， 对应的对象。2D变换又表示为傅立叶变换中的径向线。 空间，其中至少一个空间必须由任何一对中心部分共享，而 它们的确切数量是由物体的对称性决定的。正确识别（或 对于所有图像组合的公共线的近似）将允许 确定二维投影和欧拉赋值之间的角度关系 角度，这将使后续的三维重建。

This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. In single-particle electron microscopy (EM), the reconstruction of an electron density map depends fundamentally upon the assignment of Euler angles that describe the orientation of corresponding 2D projection images. A correct assignment will enable proper back-projection of the images in 3D space to create an appropriate volume, while an incorrect assignment will produce various degrees of aberrations. Thus, the challenge is identifying the true Euler angles for all projections. The problem can be solved either by directly tilting the specimen inside the microscope or through the identification of angular relationships within the 2D images themselves (for a comprehensive review, see (Frank, 2006a)). When the 2D images represent at least several views of homogeneous macromolecular complexes on the EM grid and can be described by well-defined contrast-transfer function (CTF) values, they contain all the necessary information for Euler angle assignment without requiring additional macroscopic tilting. According to the central section theorem, the 2D Fourier transform of a projection image represents a slice (central section) through the 3D Fourier transform of the corresponding object. The 2D transforms are in turn represented as radial lines in Fourier space, at least one of which will necessarily be shared by any pair of central sections, while their exact number is uniquely dictated by the object's symmetry. Proper identification (or approximation) of common lines for all combinations of images will allow for the determination of angular relationships between 2D projections and the assignment of Euler angles to each, which would enable subsequent 3D reconstruction.