%GEN TIME.RS CELL 



E XPONENTIAL GROWTH 



increase/cell: 



VOL, DNA,SOL-PROT 



GA-SYNTH.G6P-DEI >" 



TRANS, I, I- DE J 



4 3 



WT, LIPID, TOT-n} 



chit, polysac]— 

 tot-rnaI 



-[gluc|}> 



— [mel} 



Dl FFERENTIATION 



change /cell : 



incr.g6p-de, 

 lacl decrL^ 

 polysac j 



.FdECR. R- , CMP, A a1 

 [iNCR. ORG-P J 



.DECR. c<KG/|-DE 90 ^_^ |— (PROT,RNA TURNOVER] » 



DECR. 02 1 90 >r 



Fig. 9. 



A digest of some of the events which have been quanti- 

 fied during exponential growth and differentiation of an 

 RS ceU. 



POINT OF NO RETURN 



< Q. 



'H 3000 



ISOCITKIC OCKVOROCEKASC 



AGE (MRS) 



mechanism. This system is currently being 

 exploited further, and in greater depth. 



Recently, we have also turned some of our 

 attention to further exploration of the source 

 and intracellular localization of the reducing 

 power necessary for driving the reductive 

 carboxylation of ketoglutarate to isocitrate. 

 Some ten years ago. Dr. Horenstein and I found 

 that RS cells of B. emersonii possessed a poly- 

 phenol oxidase system which, in crude cell-free 

 preparations, mediated electron transfer from 

 tyrosine to either oxygen or TPN (but notDPN). 

 As was to be expected, this system could be 

 coupled in vitro with isocitric dehydrogenase to 

 drive reductive carboxylation of ketoglutarate 

 to isocitrate (Fig. 11). This tyrosinase, which 

 is not formed by the OC cell, thus constitutes 

 one source of reducingpower for the bicarbonate 

 trigger mechanism in RS morphogenesis. Un- 

 fortunately, the enzyme is firmly bound to the 

 RS wall and difficult to solubilize; thus, little 

 more has been done with it so far. 



A second source of reduced TPN in Blasto- 

 cladiella is glucose-6-phosphate dehydrogenase 

 (G6PDH). However, unlike the tyrosinase, which 



ISOCITRATE 



y\ 



SUCCINATE ^ GLYOXYLATE 



Fig. 10. 



Enzymatic reversals associated with morphological re- 

 versals in Blastocladiella emersonii. (Fig. 1, Lovett and 

 Cantlno, /. Gen. Microbiol. 24, 1961; reproduced with per- 

 mission of Cambridge University Press.) 



Fig. U. 



The two metabolic processes presumably Involved in the 

 generation of reducing power for carboxylation of a -keto- 

 glutarate. 



156 



