Cryo-electron microscopy as an investigative tool: the ribosome as an example

Cryo-electron microscopy allows the visualization of macromolecules in thei r native state. Combined with techniques of three-dimensional reconstructio n, cryo-EM images of single molecules can be used to study macromolecular i nteractions. The ribosome, a large RNA-protein complex with multiple bindin g interactions, is an excellent test case illustrating the power of these n ew techniques. Conformational changes during the binding of tRNA and protei n factors to the ribosome can now be studied without the interference of cr ystal packing. Now that the first X-ray structures of ribosomal subunits ha ve become available, conformational changes observed by cryo-EM in differen t functional states can be traced back to internal rearrangements of the un derlying structural framework. Electron microscopy, X-ray crystallography, and modeling should be used together in the endeavor to understand the func tioning of the translational machinery. (C) 2001 John Wiley & Sons, Inc.