Aspects in structural studies on ribosomes.

The intricate and essential process of the enzymatic translation of the genetic information, encoded in mRNA into proteins, is performed by the universal cellular organelle, the ribosome. Ribosomes from all organisms are assemblies of several strands of RNA and many different proteins. These components are arranged in two subunits of unequal size, which associate upon initiation of protein synthesis. A typical bacterial ribosome contains about one quarter of a million atoms and is of a molecular weight of about 2.3 million Da (1.45 and 0.85 for the large and the small subunits, respectively). About two thirds of the mass of the ribosome is composed of three chains of rRNA, the rest are some 57 different proteins (about 36 in the large and 21 in the small subunit) . Due to the fundamental significance of protein biosynthesis, ribosomes have been the target of a large number of biochemical, biophysical, and genetic studies. These studies have led to an understanding of many functional and evolutionary aspects of protein biosynthesis, provided descriptions of the overall process with varying degree of detail and led to a description of gross structural features, such as spatial in situ proximities between several ribosomal components, the secondary structure of ribosomal RNA and approximate localizations of some functional centers (for review see References 1 and 2). At the same time, the many crucial details concerning the accurate molecular mechanism that could not be revealed manifest the acute essentiality of a reliable molecular model of the ribosome. Single-crystal X-ray and neutron crystallography are currently the only experimental techniques with the potential for yielding reliable molecular models. For macromolecular assemblies of the size of the ribosome, these studies may be supported by three-dimensional image reconstruction from two-dimensional sheets. Though biological macromolecules can dynamically switch among an ensemble of conformational states, the crystallographic analysis usually reveals the structure of only one of the conformations. However, these studies may lead to the definition of a set of conformational states by