328 K. H. RdBERTS, R. ,1. HRITTKN, AM) B. J. MCCARTHY 



2. Flows a (ind h diid (Ik .iOS Xcnsomc Precursor to oOS RiUnsdiiics 



It i:? clear from the coluiun analyrsis that there exists a neusuiue pre- 

 cursor to 308 ribosomes since the peak eluted at 0.4 M contains both the 

 308 and 508 ribosomes, the radioactivity entering it is very strongly 

 delayed and rises as r- at early times. The only location in the sedi- 

 mentation analysis (Fig. 16) where such an object can occur is in the 

 308 region. Because of the lack of resolution it has been impossible 

 independently to assess its kinetics of labeling or measure its quantity. 

 After correction for the trails of 438 and eosome peaks in Fig. 16c and 

 d, it appears that the C^* radioactivity in the 308 region reaches its 

 maximum somewhat behind the P-'- peak corresponding to the 308 

 ribosomes. This result is quite uncertain but leaves an impression of 

 heterogeneity in the 308 region. 



3. Flows c and d and the SOS Precursor to 508 Ribosomes 



Figure 20 shows that there is a greater flow into the 308 region than 

 is required for the synthesis of the 308 ribosomes; in fact, more than 

 half the flow from the eosome passes into the 308 region at early times. 

 This shows that a part of the flow that ultimately reaches the 508 

 ribosome passes through a 308 neosome. We have chosen to indicate on 

 the diagram that V2 of the flow to 508 passes this way for the following 

 reasons. In the first place, the 438 radioactivity (Fig. 19) is 1/3 of the 

 neosome radioactivity (Fig. 17) at early times. In the second place, a 

 sedimentation coefficient of 308 suggests that I/2 of the 508 RNA is al- 

 ready present. It is not clear whether the two 308 neosomes indicated are 

 identical to each other. The nucleotide compositions of the 508 and 308 

 ribosomes differ but this difference could be made up through flow e. The 

 total quantity of the 308 neosomes can be crudely estimated by the 

 difference between the amount of 438 neosome (4-5%) and the total 

 amount of neosome (7%) indicated on Fig. 17. No estimate of the 

 relative quantities of the two objects indicated can be made if in fact 

 they differ. 



4. The 43s Neosome and Flows e and f 



The 438 neosome stands out clearly in sedimentation analysis (Fig. 

 16) at the appropriate time. The time course of labeling shown on Fig. 

 19 is that of the neosome or second stage in the sequence. The radioac- 

 tivity rises at t- initially and levels off later. At early times the curve 

 has the shape of ^^ (Fig. 17) but only Vr^ of the magnitude of ^.v. If 

 the total flow to the 508 ribosomes passed directly to the 438 neosome 

 from the eosome, ^^.-s would be expected to be just % of </)\. Therefore, 



