Structure and evolution of ribosomes.

Ribosomes are multicomponent particles on which biosynthesis of proteins occurs in all organisms. The best-known ribosome, namely that of E. coli, consists of three RNA's and 53 different proteins. All proteins have been isolated and characterized by chemical, physical, and immunological methods. the primary sequences of 49 E. coli ribosomal proteins have so far been determined. Studies of the shape, as well as of the secondary and tertiary structure, of the proteins are in progress. Various techniques, 3.g., immune electron microscopy and cross-linking of neighboring components in situ, give information about the architecture of the ribosomal particle. The first technique resulted in illustrative and detailed knowledge now only on the shape of the ribosomal subunits but also about the location of many proteins on the surface of the particles. The analysis of cross-links between ribosomal proteins and/or RNA's has in several cases been pursued to the level of elucidating which amino acids and/or nucleotides are cross-linked together in situ. Reconstitution of a fully active E. coli 50S ribosomal subunit from its isolated RNA and protein components can be accomplished by means of a two-step incubation procedure. From the analysis of the intermediates occurring during the reconstitution process it has been concluded that the in vitro reconstitution process resembles that in vivo assembly of 50S subunits in many respects. E. coli mutants with alterations in almost all ribosomal proteins have been isolated. Their biochemical and genetic analyses are very useful tools for obtaining information about the structure, function, and biosynthesis of ribosomes, as well as about the location of the genes for these proteins on the chromosome. From comparative electrophoretic, immunological, protein-chemical, and reconstitution studies on ribosomes from various species it has become clear that their is little homology between ribosomal proteins from prokaryotes and those from eukaryotes. This finding is surprising since there is no essential difference in the way in which pro-and eukaryotic ribosomes function in protein biosynthesis.