represented both by the change in base com- 

 position (Table II, lower part) and by a clear 

 coincidence of the counts with the absorbancy 

 pattern. (Notice the constant specific activity 

 across the ribosomal optical density peak in 

 Fig, 5.) Thus RNA synthesis begins in this 

 system under conditions such that little or no 

 ribosomal RNA is made and the major incor- 

 poration activity represents labeling of the CCA 

 terminal in transfer RNA. In time, the rate of 

 end-labeling falls and the rate of synthesis of 

 heavy heterogeneous RNA rises steadily from 

 fertilization onward. At some point, probably 

 well after the blastula and perhaps as late 

 as the time of appearance of definitive nucleoli, 

 the synthesis of ribosomal RNA begins in 

 quantity. This means that a complicated system 

 of control operates on the synthesis of RNA, and 

 specifically, on the utilization of the cistrons 

 that provide templates for synthesis of the 

 ribosomal RNA. 



This is all heavily descriptive, and I cannot 

 offer anything in the way of a reasonable ex- 

 planation for the existence of this pattern, but 

 it is beginning to be quite a general one. For 

 example, the situation in the amphibian seems 

 to be roughly the same, except that there is 

 some argument about when the synthesis of 



new heterogeneous RNA begins. Dr. Kohne will 

 tell you about this later. 



POLLARD: I would like to ask you a couple 

 of questions. First, as a microbiologist, I'd 

 like to know what the amount of turnover of 

 RNA and protein is. In E. coli, for example, 

 the RNA does turn over to some extent. At least 

 the uracil label changes. 



GROSS: Does it turn over in the ribosomal 

 RNA? 



POLLARD: I don't think it does, but if we 

 just look at the general cell behavior, there is 

 a difference between thymine label and uracil 

 label. To what extent do you see something like 

 that? 



GROSS: There are two different answers, 

 depending on how you evaluate the available 

 data. Comb (8) believes that there is some 

 considerable degradation of ribosomal RNA 

 in sea urchin embryos from the beginning of 

 development to the gastrula stage, and that 

 the products of degradation are possibly used 

 for resynthesis of messenger RNA. That is the 

 only information I know that suggests such 

 a turnover. Other data that we have do not offer 

 much support for this idea. The egg starts its 

 life with a large pool of precursors for RNA, 

 and this pool diminishes slowly but steadily 



