VII. SYNTHESIS OF RNA AND RIBOSOMES 329 



it is clear that about half the flow to 50S ribosonies passes to the 43S 

 neosome from the eosome. 



It appears certain that no eosomal RNA goes directly to the SOS 

 ribosome because <^5o is proportional to t^ at early times. The 43S 

 neosome is shown on the diagram (Fig. 21) as containing the full 

 complement of RNA of the oOS ribosome. This seems likely from its 

 sedimentation constant. 



The quantity of 43S neosomal RNA can be estimated to be about 

 5% from the steady-state P^- radioactivity that remains in this region 

 after the 308 and oOS contributions have been subtracted, assuming 

 reasonable and symmetrical peak shapes. The specific radioactivity esti- 

 mated on this basis at early times is just V2 what would be expected if 

 the total flow to 50S ribosomes passed directly from the neosome to the 

 43S. This is, of course, consistent with the diagram since % of the flow 

 should be delayed by the 308 neosome precursor to the 438. 



The diagram (Fig. 21) shows a set of sequential relationships which 

 are more complex than the two sequential precursors shown on Fig. 17. 

 The diagram suggests that half of the radioactivity of the 508 ribosomes 

 should rise in proportion to t* for a time while the other half should 

 rise as t^. All of the 308 ribosome radioactivity should rise as t^. 



While deviations from the three-stage model showed clearly in the 

 time course of labeling precursors such as the 308 and 438 neosomes, the 

 accuracy of the data is not adequate to resolve the predicted fourth 

 power component in the final product. 



These studies of the synthesis of the RNA portion of ribosomes 

 provide only a few hints to the concurrent process of the addition of 

 protein. The step from 438 neosome to 508 ribosome involves both a 

 change in sedimentation coefficient and a change in the elution from 

 DEAE, but no additional RNA is added. It seems quite obvious that 

 this change is due to a change in the protein content. Studies using 

 C^*-leucine to follow the synthesis of the protein are described in Section 

 VII. 



VI. Kinetic Studies of Phenol-Extracted RNA 



The kinetics of incorporation of tracers can also be observed in the 

 purified RNA extracted by phenol. After short exposures to C"-uracil 

 some radioactivity is associated with the main peaks of 168 and 238 

 RNA and the rest appears as a component of average sedimentation 

 coefficient of about 88 (McCarthy and Aronson, 1961; Gros ef al., 1961). 

 At the earliest times, however, there is always a considerable fraction 

 of the radioactivity of sedimentation coefficient of 168 or greater even 

 though the specific radioactivity of the 88 region is high (Fig. 22). 



