and the total counts decrease. All the loss of 

 label from the plant can be accounted for by the 

 label that reappears in the medium. Our inter- 

 pretation is that part of the problem with uracil 

 incorporation in our previous experiments was 

 that we are trying to go against the system. Dur- 

 ing the stage from 16-17 hr, not only is the plant 

 degrading RNA, as shown in one of the earlier 

 figures, but the pools themselves appear to be 

 actually shrinking at this time. Both factors 

 would work against uracil penetration. 



Figure 14 represents an experiment where 

 we tried to circumvent this difficulty. I'm not 

 entirely sure whether we did or not, but I think 

 we did in part. The experiment was based on an 

 assumption with which you may not agree and 

 which we have to prove: that the "heavy" RNA 

 was predominantly a ribosomal precursor. My 

 argument will have validity only as far as this 

 is true. We grew the cells as usual, adding Ci"*- 

 uracil for 1 hr during the exponential growth to 

 randomly label the whole- cell RNA. Then, as a 

 function of time, we pulsed the cells with tritium- 

 labeled uracil to see how much could enter RNA 

 from the outside. The pre-existing, randomly 

 labeled RNA inside the plants should give us 



O 



Q. 

 O 



I 



I 



I 



I 



some idea of the activity within the cells, par- 

 ticularly at the later stages when we could not 

 get at it by pulsing from outside. As you can 

 see (Fig. 15), the pattern of the randomly 

 labeled RNA remains essentially constant 

 throughout. However, the pulse labeling with 

 tritium is very similar to our previous results 

 with C''* -uracil: rather disperse labeling and a 

 high-specific activity, heavy peak which com- 

 pletely disappears by 17-17}^hr. No carbonic 

 activity appears in this region, even though we 

 know from all our previous evidence that a con- 

 siderable amount of this RNA is degraded. If 

 one accepts the idea that the heavy peak is a 



10 30 10 



FRACTION NUMBER 



Fig. 15. 



30 



Sedimentation profiles of steady-state labeled (C'^ ) and 

 pulse- labeled (H^) RNA vs time. 



16 



HOURS 



19 



HOURS 



Fig. 14. 



Incorporation and distribution of C 1* -uracil label vs 

 dme. 



Fig. 16. 

 Acdnomycin inhibition of uracil Incorporation. 



172 



