A New Approach for Building an Atomic Model from a Three-Dimensional Electron Microscopy Data

*We have developed a new approach to build an atomic model from a low-resolution three-dimensional electron microscopy data for analyzing the structures of 70S ribosome. In this approach, a huge number of atomic models with different conformations are prepared first, and then the best-fitting atomic model is selected from them for each electron microscopy structure. The rigid-body fitting calculation, in which the atomic model is moved as a rigid body by translational and rotational manipulation, is most intensively performed. However, the calculation takes quite long if all the six rigid-body parameters are treated as variables at the same time. Thus, we perform two different calculations alternatively, in which only three parameters are treated as variables and the others are fixed. We have ascertained that the calculation is fast and accurate enough.