Percent generation time 



The pattern for net synthesis of different RNA's in an R.S. PLArsrr ouRrNO 



DIFFEREmiATION. 



Age, hr 



The MOLAR COMPOSITION OF RNAn.Q ► 



Values for RNA arc expressed as total /iMotes of aJI nucleotides derived from the RNA by Isolated at different stages during differentiation of an R S plant of B emersonii 

 KOH hydrolysis. 



Fig. 19. 



Transformations In the extractablllty and composition of the RNA of Blastocladiella emer- 

 sonii during RS differentiation. (Figs. 2, 3, Cantino, Phytochemistry 1, 1961; reproduced 

 with permission of Pergamon Press Limited.) 



change in some way so that the system is modu- 

 lated and the products are different. Now, it 

 seems to me that in both of these there is a 

 critical way to determine whether that is the 

 whole story. That is to find out if at some point 

 before the point of no return you can disable the 

 genome. You disable the genes so that no infor- 

 mation can flow from them. If in that instance 

 this process continues, it's clear that this is a 

 modulation. It' s differentiation, to be sure, but 

 without benefit of gene action. Now, on the other 

 hand, if, in the condition where we stop the gene 

 action, this too stops, then you know you've got 

 to have something additional to the simple modu- 

 lation of the rates of enzymatic processes and 

 the way that they interact. 



CANTINO: I'm inclined to agree (but I have 

 no direct evidence which will bear on this point). 

 However, may I summarize some analytical 

 results obtained in 1960 which may have an 

 indirect bearing here? (The following reply is 

 an expanded version of the original one; the 

 data can be found in ref. 4). Essentially all of 

 the RNA in a growing RS cell is soluble in hot 

 NaCl (= RNANaci-soi ), and it's molar com- 

 position (CMP:AMP:UMP:GMP = 1.00:1.33:1.08: 

 1.27) stays constant during the last stages of 

 exponential growth before the point of no return 

 is reached. But, after the point of no return, the 

 quantity of RNAwaci-soi per cell begins to de- 



crease again, and simultaneously it undergoes a 

 sharp change in composition (Fig. 19). Just be- 

 fore the amount per cell of this NaCl-soluble 

 RNA reaches its peak, a new RNA appears in the 

 cell. This new RNA is attached to cell particles 

 which sediment at 10,000 to 15,000 x G, it is in- 

 soluble in hot NaCl but detectable by KOH hy- 

 drolysis to yield its component nucleotides, and 

 these nucleotides are present in it in almost 

 exactly equal quantities (CMP:AMP:UMP:GMP = 

 1.00:1.00:1.00:1.03). This insoluble RNA 

 (RNA insoi ), as seen in the figure, rises sharply 

 in the cell immediately after the point of no 

 return, and it reaches its maximum level when 

 the cell is about 70 hr. old. The plot for RNA total 

 in the figure was simply gotten by adding up the 

 data for RNAinsoi and RNA Naci-soi . Assays 

 for these two RNA types were also made in RS 

 cells which had been induced to undergo morpho- 

 logical reversal. The data (not shown in this 

 figure) revealed that just before the point of no 

 return, when RNA insoi first becomes detect- 

 able, morphogenetic reversal induced a sud- 

 den loss of about half of this RNA insoi • Similar 

 experiments performed after the point of no 

 return did not induce the shift. 



DEERING: I have something that might be 

 relevant to this. I've started some work on the 

 effects of ultraviolet light on the OC develop- 

 ment of this organism. Ultraviolet light is be- 



163 



