during development. It doesn't enlarge, as far 

 as I can tell, at any time. Actual synthesis of 

 bulk RNA, represented at least by the incor- 

 poration of radioactivity, seems to be small, 

 perhaps not exceeding a few per cent of the 

 original total. Certainly on this basis there 

 is no need for massive synthesis of bulk RNA. 

 Finally, by a technique that I'll describe in a 

 minute, we have been able to label the RNA 

 in ribosomes of unfertilized eggs. If such 

 eggs are fertilized and allowed to develop in 

 the presence of a very large excess of un- 

 labeled uridine and cytidine in the medium, 

 there is no detectable loss of counts in the 

 RNA. This is probably in inadequate "chase" 

 and certainly not direct evidence for the com- 

 plete stability of cytoplasmic RNA. There is, 

 in short, no really adequate answer to your 

 question at this moment, but one's prejudice 

 is in the direction of little or no turnover. 



POLLARD: How about the case of protein 

 synthesis? 



GROSS: That may really be the more in- 

 teresting matter. The egg starts its life with a 

 large pool of amino acids, but a peculiar one 

 because of its abnormal composition relative 

 to that of typical proteins. A very large fraction 

 of the osmolarity of the sea urchin egg is pro- 

 vided by glycine. Some of the other amino 

 acids, such as leucine, are in short supply in 

 the pool. In any case, there is such a pool, but 

 I would guess that it is probably not adequate 

 for prolonged synthesis of a variety of proteins, 

 such as begins at the beginning of development. 

 Now, I cannot tell you what the real rate of 

 protein synthesis is following fertilization, be- 

 cause we don't have proper information about 

 the pool changes. The total protein of the egg 

 does fall by about 15% from fertilization until 

 the larval stage. However, the egg has in it a 

 very large amount of yolk, most of which is 

 gone by the end of the larval period. This yolk 

 is mostly protein, 90% or so. Consequently, 

 there is a very significant transformation of 

 protein. There must be a lot of traffic through 

 the pool, most of it being provided by yolk at 

 one end and stable new proteins at the other. 



KOHNE: May I make a comment on the 

 first question? In these embryos, it is very 

 difficult to do quantative studies and it would 

 be very difficult to determine turnover if it 

 were occurring. 



GROSS: Yes, most of what I said is per- 

 haps circumlocution. We don't have adequate 

 pool data, nor suitable "chase" techniques for 

 obtaining a satisfactory answer to the question. 



POLLARD: The protein turnover with re- 

 spect to yolk wasn't circumlocution, was it? 



GROSS: No. It is clear that some protein 

 disappears and new protein forms. Perhaps in 

 the course of the discussion, we can come back 

 to yolk. 



GRUN: Is there an obvious simple explana- 

 tion for this inverse relationship between the 

 specific activity curves and the OD curves shown 

 in Fig. 4? 



GROSS: Suppose that the counts in each 

 fraction were invariant, so that the computation 

 of a specific activity involved a division at each 

 gradient point of a constant number by a variable. 

 The variable number, i.e., the optical density, 

 is alternately high and low. Where it is low, 

 you get a high value of specific activity, and 

 where it is high, you get a low value. Therefore, 

 in such an ideal case, with radioactivity a 

 straight line of zero slope throughout the gradi- 

 ent, the computed radioactivity would be a 

 pattern exactly the inverse of the optical densi- 

 ties. Now, as the actual radioactivities deviate 

 from that ideal condition, the oscillations in the 

 computed specific activity will be damped, and 

 when the optical density and radioactivity are 

 completely coincident, the computed specific 

 activity becomes a straight line. I'm about to 

 raise this point again in connection with radio- 

 activity in the unfertilized egg. 



HYMER: May I ask about the high specific 

 activity in the region of the gradient containing 

 molecules larger than 28S? Is there evidence 

 for a heavy ribosomal precursor molecule in 

 your system? 



GROSS: No, there isn't. Certainly not at the 

 beginning of development when there isn't any 

 evidence of accumulating ribosomal RNA. 



KOHNE: A "heavy" ribosomal precursor 

 has been demonstrated in Xenopus laevis. 



MASSARO: I'd like to deviate from the 

 subject for a second. During the early protein 

 synthesis, what contribution is the male com- 

 ponent making? From this type of analysis, 

 we're developing a type of parthenogenetic 

 embryo. 



GROSS: There is no difference in the pat- 

 tern of protein synthesis - at least that we can 

 determine by methods that I'll discuss later - 

 between a parthenogenetic merogone, which has 

 neither a sperm nucleus nor an egg nucleus, 

 and the fertilized egg during the first few 

 cleavages. 



KOHNE: What is very interesting is that 

 such an active process could go on at the low 

 levels of sRNA present in eggs. 



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