RNA IN THE SYNTHESIS OF PROTEINS 



CD 



2(30S) 



2(50S) 



2(70S) 



K100S) 



M.W. x 10 0.85±0.15 1.80 ±0.15 2.8 ± 02 5.9 ± 1.0 



All particles are composed of 64 °/o RNA and 36°/o protein 



Fig. I. Diagrammatic representation of E. coli ribosomc sub-units and their aggrega- 

 tion products. (The molecular weight data are from Tissieres et al. ls ) 



Fig. 2. Electron micrograph of negatively stained E. coli ribosomes (Huxley and Zu- 

 bay 15 ). Two particle types are predominant: (i) 70s containing two sub-units of 

 unequal size, and (2) 100s consisting of two 70s ribosomes joined together at their 



smaller (30s) sub-units. 



combine to form a 70s ribosomc. At still higher Mg ++ concentrations, two 

 70s ribosomes dimerize to form a 100s ribosomc. (Figs. 1 and 2). 



Ribosomes from every cellular source have a similar sub-unit construction. 

 As with E. coli ribosomes, the level of divalent cations determines which 

 ribosomes exist. Bacterial ribosomes seem to require higher Mg ++ levels in 

 order to aggregate into the larger sizes. Conversely they break down much 



s-115 



