324 



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



appears in the 14S region and the shape of the curve indicates that the 

 total flow of material to ribosomal RNA passes through this stage. The 

 curve for ^k-, has zero slope at zero time showing that the entry of 

 radioactivity is delayed by a precursor and that little, if any, C^^-uracil 

 by-passes the precursor. The values of <j)ir at early times indicate a 



0.1 



0.001 



0.0001 



0.001 0.01 1 0.1 1.0 



TIME -T 

 ( Defined by growth Q = QqS ^ ) 



Fig. 19. Log-log plot of the newly synthesized fraction of RNA present in the 

 total cell e.\tract (#), eosome (A), 43S neosome (D), and 50S ribosome (O) as a 

 function of time. Data from Fig. 16. The curve drawn for the SOS ribosomes is the 

 calculated curve shown on Fig. 17 multiplied by 2/3 since the SOS ribo.somes account 

 for 2/3 of the ribo.«omal nucleoprutcin. The agreement is striking. 



precursor quantity of 3.5%. The quantity of precursor estimated from its 

 final level is 2%. Although this discrepancy may indicate complexity of 

 the first precursor, the material which sediments in the 14S region can be 

 identified with the eosome fraction resolved by chromatography. 



Figure 19 shows ^^ plotted together with 04,j, ^-,„ and 4>r- At early 

 times 0,.i i.s proportional to r- and <^,-,o is proportional to t'\ At late times 



