TABLE V 



Stimulation by G6P and Mg 



+ 2 



Additions 



None 



G6P (2 X 10" 3m) 



MgCl2 (1 X 10- 3m) 



G6P + MgCl 



261 485 



973 2,158 



309 603 



1,508 2,317 



TABLE VI 



Stimulation of Polymer Synthesis by Trehalose ' 



Additions c .p. m. 



* From Wright, Barbara, E.: Control of Carbohydrate 

 Synthesis in the Slime Mold. In Developmental and Meta- 

 bolic Control Mechanisms and Neoplasia (A Collection of 

 Papers Presented at the Nineteenth Annual Symposium 

 on Fundamental Cancer Research, 1965), p. 312. Balti- 

 more, The Williams and WiUcins Company, 1965. 



ably be a limiting factor at culmination for cell 

 wall synthesis. 



Table VII shows a complex relationship 

 between UDPG concentration and glucose -6- 

 phosphate concentration in their effect on cell 

 wall synthesis. It can be seen that G-6-P only 

 stimulates cell wall synthesis at a low level of 

 UDPG, but not at a high level. Therefore, their 

 effects are interdependent. Although it isn't 

 shown in this table, the concentration of G-6-P 

 which maximally stimulates is about 10-3 M and 

 in the intact cell it never reaches a level higher 

 than 10 ""^M. Glucose-6-phosphate would, there- 

 fore, presumably be limiting in the cell for cell 

 wall synthesis. For unknown reasons intracellu- 

 lar UDPG levels vary significantly from one 

 stage study to another. Although the maximum 

 concentration is always at culmination, the level 

 throughout differentiation in a particular stage 

 study may be unusually high or unusually low. 

 Thus, G-6-P could serve as a buffering agent, 

 exerting strong stimulation at low UDPG levels 

 and less stimulation in cells which are not as 

 limited with respect to their UDPG levels. 



It is apparent from these and many other 

 studies that the existence in vivo of many limit- 

 ing factors for the synthesis of materials im- 

 portant to differentiation may be the rule rather 

 than the exception. It is known that even in fully 

 differentiated cells enzymes are usually operat- 

 ing far below their potential activity due to sub- 

 strate limitation. Cells undergoing differentia- 

 tion are frequently dependent entirely upon 

 endogenous metabolism and have very limited 

 resources from which to obtain the necessary 

 energy and building blocks for the many syn- 

 thetic processes required in morphogenesis. If, 

 in fact, it is true that multiple limiting factors 



considerably. It is possible, therefore, that mag- 

 nesium is limiting cell wall synthesis during 

 development. In the experiment shown in Table V 

 the enzyme was prepared on day 1 and assayed 

 immediately in the presenceof UDPG- i**C; when 

 the enzyme was aged for a day and assayed on 

 day 2, it had become activated, but the require- 

 ments remained comparable. I'll discuss activa- 

 tion later. 



Table VI shows that trehalose stimulates 

 cell wall synthesis, but only in the presence 

 of G-6-P. We don't understand the mechanism 

 for trehalose stimulation, but I want to make the 

 point that trehalose, according to Filosa, accu- 

 mulates very late in development during soro- 

 carp construction, so that trehalose would prob- 



TABLE VU 

 Interdependence of G-6-P and UDPG 



114 



