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



TABLE VI 



CllANCiK IN AlTAUKNT CoMTOSITIUN OF UNA DePE.NOING 



UN Pkkiou of KxrosiRK to V^'K)^ 



C. FRACTIONATION 



At early times after the addition of the tracer the bulk of the P^- is 

 still in the eosome fraction. Thus the composition observed at early 

 times should be a measure of the composition of eosomes. Ribosomes free 

 of S-RNA can be obtained by several cycles of centrifugation in tris 

 buffer containing 10"- M Mg. The entire eosome fraction can be released 

 from the ribosomes by reducing the magnesium concentration to 10 ■* M. 



TABLE VII 

 Fractionation of Newly Formed RNA" 



Released by Released by Not released 



10-^ Mg H2O by H2O 



C23,2 25.0 215 



A 25.0 24.4 25.3 



G29.8 27.0 32.8 



U21.8 23.6 20.4 



» A pellet of 70S ribosomes was prepared from cells given 2 minutes' exposure to 

 P32O4 . The pellet was then suspended in buffer with a magnesium concentration of 

 lO"** M or in water with magnesium concentration of 3 X 10^' -U and centrifugod 40K 

 240 minutes to remove ribosomes and adhering RNA. 



