The ribosome through the looking glass.

For almost 20 years crystallographers have sought to solve the structure of the ribosome, the largest and most complicated RNA-protein complex in the cell. All ribosomes are composed of a large and small subunit which for the humble bacterial ribosome comprise more than 4000 ribonucleotides, 54 different proteins, and have a molecular mass totaling over 2.5 million Daltons. The past few years have seen the resolution of structures at the atomic level for both large and small subunits and of the complete 70S ribosome from Thermus thermophilus at a resolution of 5.5-A. Soaking of small ligands (such as antibiotics, substrate analogues, and small translational factors) into the crystals of the subunits has revolutionized our understanding of the central functions of the ribosome. Coupled with the power of cryo-electron microscopic studies of translation complexes, a collection of snap-shots is accumulating, which can be assembled to create a likely motion picture of the bacterial ribosome during translation. Recent analyses show yeast ribosomes have a remarkable structural similarity to bacterial ribosomes. This Review aims to follow the bacterial ribosome through each sequential "frame" of the translation cycle, emphasizing at each point the features that are found in all organisms.