VII. SYNTHESIS OF RNA AND RIBOSOMES 317 



tration of 10- M, which is necessary for the preservation of large ribo- 

 somes. At early times the specific radioactivities of the 308 and 50S 

 ribosomes present in the extract were considerably higher than that of 

 the 70S. These small particles, described as native 30S and 508 to dif- 

 ferentiate them from those derived by breakdown of the 70S at lower 

 magnesium concentrations, represent together some 10 to 20% of the 

 ribosomal material. Their higher specific activity after a pulse was 

 interpreted as showing a precursor relationship to the 70S ribosomes. 



Figure 14 shows the analysis of extracts of four samples of cells 

 taken after 10, 20, 50, and 55 minutes' exposure to C^^-uracil. 508 and 

 308 ribosomes show as shoulders in the ultraviolet absorption profile. 

 At 10 minutes the specific radioactivities of the 508 and 30S ribosomes 

 are at least three times that of the 70S. This difference in specific radio- 

 activity decreases with time and has practically disappeared by 55 

 minutes. 



When a series of cell extracts was analyzed after much shorter 

 exposures to C^^-uracil the results were not interpretable in terms of a 

 simple hypothesis. At early times an appreciable fraction of the radio- 

 activity appeared in the 708 particles. The quantity was greater than 

 would be expected in an end product. This suggested direct entry of 

 RNA into 708 without the delay brought about by passing through the 

 large pools of 508 and 308 precursors. Another possibility was that the 

 labeled RNA was loosely associated with the 708 peak rather than being 

 incorporated into the large ribosomes. 



Because of the complexity encountered in the analysis of cell extracts 

 made in 10"- M Mg*"", a search was made for conditions more suitable 

 for the resolution of precursors from products in ribosome synthesis. 

 Comparison was made of a pulse labeled cell extract prepared and 

 analyzed in a range of magnesium concentrations (Fig. 15). 



At 10'- M Mg*"^ (Fig. 15a) most of the ribosomes are present as 70S 

 but most of the radioactivity runs as 508 and 308. The apparent specific 

 radioactivity of the small ribosomes is more than three times that of the 

 70S. Cells broken in the presence of 3 X 10"' M Mg++ (Fig. 15b) have 

 most of their ribosomes as 508 and 30S. At the same time about half 

 of the radioactivity associated with ribosomes in Fig. 15a now appears in 

 a peak of about 148. There is now little difference between the specific 

 radioactivities of the three groups of ribosomes although there is a peak 

 of radioactivity running between the main 508 and 30S peaks. In 10'^ M 

 magnesium (Fig. 15c) an even higher proportion of the radioactivity 

 appears in a 148 peak, and the faster of the other two peaks of radioac- 

 tivity appears to lag behind the 508 ribosome peak. The distribution of 

 radioactivity between the three objects is very similar at 10^^ M Mg^* 



