Alternative Views on the Ribosome Cycle

Abstract

INTRODUCTION
While earlier studies of organisms, which were limited to their visible features, emphasized diversity, studies at a molecular level have increasingly revealed the underlying unity. This development has been exceedingly gratifying, for it has converted a mass of empirical biochemical data into direct evidence for the evolutionary continuity of the living world. Yet at the molecular level diversity is also encountered, especially in regulatory mechanisms, and it is just as inevitable a consequence of evolution, and just as interesting, as is uniformity. Hence the existence of extensive biochemical uniformity should not tempt us to try to force all metabolic processes into a Procrustean bed fitted for E. coli.

Protein synthesis illustrates this point. It exhibits striking unity between prokaryotes and eukaryotes in the details of the microcycle of chain elongation, but the process of initiation seems to be less uniform (for example, in formylation)—perhaps because it is a site of regulation. In particular, there are grounds for questioning Kaempfer’s proposal (this volume) that eukaryotes and prokaryotes have the same ribosomal macrocycle. His model, which involves unstable termination ribosomes and stable reserve ribosomes, fits the data so far available for eukaryotes; but with bacteria, there is considerable evidence for a simpler scheme in which there is only one kind of free runoff ribosome, and it is in rapid spontaneous equilibrium with free subunits. This paper will review that evidence and will comment briefly on certain historical aspects of the ribosome cycle.

Subunit Exchange between Ribosomes after Runoff
The key...