VII. SYNTHESIS OF RNA AND RIBOSOMES 



297 



ribosomes in distinction to the "derived" SOS and 50S particles which are 

 obtained from the dissociation of 70S particles. Native particles have 

 higher specific radioactivities (after a short period of exposure to the 

 tracer) than do the derived particles (Britten and McCarthy, 1959). 

 Isolated native particles do not combine (Green and Hall, 1961). 



uj 



50 85 



C D 



Fig. 4. Changes in ribosome pattern caused by different metabolic conditions: 

 A. The lOOS component is prominent when growth is stopped by lack of energy 

 source. B. Magnesium starvation causes disappearance of ribosomes. C. Incubation 

 with chloramphenicol results in accumulation of small particles. Ribosomes already 

 present appear as 70S. D. Incubation with 5-fluorouracil results in accumulation of 

 308 and 50S ribosomes. 



Furthermore, it has been shown m vivo (McQuillen et al., 1959) and 

 in vitro (Tissieres et al., 1960) that the 70S particles carrying newly 

 made protein do not dissociate completely when the magnesium concen- 

 tration is reduced to 10 ■* M. 



In addition to the main peaks which are readily observed smaller 

 groups of particles sometimes appear. Many preparations show a peak 

 at about 20S. The 20vS component is complex, containing one component 



