840 



R. B. ROBERTS, R. J. BRITTEN, AND B. J. MCCARTHY 



Further centrifugation sediments tlie ribosomes leaving the eosomes in 

 the supernatant fluid. Chromatography on DEAE provides a final puri- 

 fication of the eosome fraction (Fig. 29). The composition of the material 

 obtained by this procedure is intermediate between DNA and RNA as 

 was expected. 



If, however, the ribosome pellet is suspended in water containing a 

 slightly higher concentration of Mg only a portion of the eosome 

 fraction is released. Centrifugation then separates the free eosomes from 

 those still adhering to ribosomes. The compositions of these two fractions 

 differ; the free eosomes are more like DNA and those still adhering to 

 the ribosomes are more like the final RNA product (Table VII) 

 (Midgley, 1961). 



D. RNA CLASSES 



The compositions of the 308 and 50S ribosomes and the apparent 

 composition of the newly formed RNA show some systematic relation- 

 ships. These can be interpreted as indicating that the different types of 

 RNA are the result of different proportions of two hypothetical RNA's 



Fig. 30. Relationship between composition of DNA and various RNA fractions. 

 The composition of DNA is plotted at the loft (D). The compositions of early 

 labeled RNA (E), 308 ribosomal RNA (30), and 50S ribosomal RNA (50) are 

 plotted at the positions expected from the hypothesis described in the text. At the 

 right the lines extrapolate to the composition of a hypothetical purinc-rich 

 RNA (R). 



