VII. SYNTHESIS OF RNA AND RIBOSOMES 



327 



that three main stages may be described. The clearest demonstration 

 derives from the rate of labeling of the main ribonucleoprotein peak in 

 the column analysis. Its radioactivity rises initially as the cube of time. 

 The total label present at early times is so low as to require two sequen- 

 tial precursors. The total precursor material can be only 10% of the 

 total RNA and a single precursor of this magnitude could not introduce 

 such a delay into the product. The diagram shown in Fig. 21 summarizes 



DEAE COLUMN ANALYSIS 



Molarity of NoCI at peak 



0.6 Q5 0.4 



Fig. 21. Schematic diagram of the flow of RNA in ribosomal synthesis. The 

 diagram indicates the way in which the information derived from DEAE-cellulose 

 column analysis and sedimentation analysis has been combined to give a more com- 

 plete scheme. The figures in the boxes represent the percentages of the total ribo- 

 somal RNA existing in each of the states in a steadily growing cell. Movement 

 downward is a re.sult, principally, of the completion of RNA subunits. Movement to 

 the right is principally a result of the addition of protein. 



the results obtained from chromatography and sedimentation and indi- 

 cates the correlation between the two analyses. In the following para- 

 graphs the evidence which supports each of the features of this diagram 

 will be considered. 



1. Eosome 



The interpretation of the role of the eosome as precursor to all of the 

 ribosomal RNA has been discussed above. Both the peak at 0.6 M in the 

 column analysis and the broad region peaking at 14S in the sedimenta- 

 tion analysis carry all of the radioactivity at early times and later 

 saturate at about the same quantity of radioactivity. 



