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



early times the 43S coinponent aceounted only fur 1/3 of the neosome 

 fraction. One interi)retation of these results is that the BOS peak inchules 

 not only the SOS neosome static of the SOS ribosome but also a SOS 

 neosome precursor to the 4SS. 



D. STAGES OF SYNTHESIS OF THE RIBOSOMAL RXA 



From the analysis of uracil-labeled cell extracts, it is clear that at 

 early times all the radioactivity is present in a fraction, named eosome, 

 peaking at 14S. The kinetic behavior of the com])onent has been studied 

 and its size estimated at 2-37^ of the total ribosomal RNA. It can be 

 shown from the same experiments that label entering nucleojirotein of 

 higher molecular weight is delayed by a component of about the same 

 size. Together these observations suggest that the eosome is predomi- 

 nantly precursor to ribosomes. 



In another recent study (Gros et al., 1961) the rapidity of labeling 

 of the eosome has been invoked as a criterion of turnover. These authors 

 have suggested that the rapid rate with which tracer enters and leaves 

 the eosome fraction proves instability of these RNA molecules. In gen- 

 eral, however, it is clear that the rapid labeling of a small component 

 indicates merely that the flow into it is much larger than that required 

 to maintain its quantity in the growing cell. The loss of label when an 

 excess of unlabeled isotope is added shows only the already obvious fact 

 that there is also a transfer of material out of this component. These 

 measurements cannot be expected to distinguish between a precursor 

 through which label flows to a product and a compound which is synthe- 

 sized and then is broken down to its constituent parts. What we have 

 demonstrated is that there exists a precursor-product relationship be- 

 tween the eosome and the product ribosomes for the transfer of C"- 

 uracil radioactivity. 



This does not prove that labeled molecules initially observed as 

 eosomes are incorporated as such into completed ribosomes. An alterna- 

 tive involves breakdown of the eosome molecules and a synthesis of 

 ribosomes by means of a quantitative reutilization of the labeled degra- 

 dation products. It should be pointed out that, in common with many 

 similar tracer studies, the latter alternative cannot be distinguished. For 

 quantitative reutilization the breakdown has to occur in such a way that 

 there is no mixing with other RNA precursors. The kinetics of tracer 

 flow are most simply interpreted on the basis of precursor product 

 relationships, although other evidence discussed in Section VIII points 

 to greater complexity. Here we follow the simple interpretation as a 

 first approximation. 



As far as the details of ribosome synthesis are concerned, it is clear 



