The Ribosome Cycle

Abstract

INTRODUCTION
Three forms of ribosomal particles are generally observed in extracts of both prokaryotic and eukaryotic cells: polysomes, single ribosomes and ribosomal subunits. Analysis of the functional relationship between these three classes and their role in the ribosome cycle in protein synthesis has been complicated chiefly for two reasons. First, a complete turn of the ribosome cycle—repeated passage of a ribosome over messenger RNA—requires the participation of so many components that faithful reproduction in a purified, reconstituted cell-free system is virtually impossible. Individual steps can be analyzed more precisely, but their placement within the cycle is often problematical. Second, the three observed states of ribosomal particles are stable enough to be recovered in extracts, but the existence of additional, less stable states can only be revealed by special or indirect methods. Moreover, particles in different states may not always be separated by ultracentrifugation. These complications at times have led to apparently contradictory results.

To simplify the analysis of available facts, I shall present a general scheme for the ribosome cycle, discuss the evidence in support of its individual features, show how the available observations in bacterial and eukaryotic systems can be accommodated within this scheme, and finally present the alternative interpretations to which the scheme remains open.

THE RIBOSOME CYCLE—A GENERAL SCHEME
Several ribosomes are engaged in protein synthesis as they move along a messenger RNA molecule, forming a polysome (depicted at the top of Figure 1). At the termination codon (t), the completed polypeptide chain is...