7b CONTROL MECHANISMS IN CELLULAR PROCESSES 



Wijusundra and Woods ( 1953, 1960 ) on the synthesis of methionine 

 by E. coli. Both groups observed essentially that E. coli cells har- 

 vested from a glucose-mineral salts medium could convert homo- 

 cysteine to methionine in the presence of serine and p-amino benzoic 

 acid. However, if methionine had been incorporated into that me- 

 dium, the harvested cells were unable to perform this conversion. 

 In another example in E. coli, Vogel and Davis (1952) observed 

 that when the cells of an arginine auxotroph blocked earlier than 

 ornithine had been previously grown in an arginine medium, growth 

 in an ornithine or citrulline medium occurred only after a lag period. 

 In contrast, ornithine grown cells grew in media containing ornithine, 

 citrulline, or arginine without lag. 



Although the conversion of homocysteine to methionine has re- 

 cently been undergoing a vigorous enzymatic analysis (Hatch et al., 

 1959; Szulmajster and Woods, 1960), the missing enzyme in the 

 methionine grown cells has not been revealed. In contrast, not only 

 has the repression of the arginine synthesizing enzymes in arginine 

 grown cells been demonstrated (Vogel, 1956; Gorini and Maas, 1957), 

 but, as shown in the chapter by Vogel ( 1961 ), this pathway has sup- 

 plied the model par excellence for the study of enzyme repression 

 in a biosynthetic pathway. 



An example of the evidence supporting the view that the enzymes 

 might be present but not functioning was described by Brooke et al. 

 ( 1954 ) . They employed a uracil auxotroph of A. aerogenes which 

 was blocked after orotic acid and which accumulated this compound. 

 Cells of this mutant did not accumulate orotic acid in a medium con- 

 taining excess uracil. However, upon transfer to a uracil free me- 

 dium, orotic acid accumulation began immediately. Thus, uracil 

 liad prevented the action of an enzymatic sequence already present 

 in the cells. Similarly, in view of the way in which the isotope 

 competition experiments cited above were performed, it must be 

 concluded that the cells contained the necessary enzymes for isoleu- 

 cine biosynthesis (wild-type cells harvested from minimal medium). 

 Yet upon transfer to an unlabeled isoleucine-labeled glucose me- 

 dium, it was apparent that the pathway from glucose to isoleucine 

 was completely and immediately blocked. 



An opportunity to study the mechanism of this block between 

 glucose and isoleucine in E. coli presented itself while the first step 

 leading specifically to isoleucine was being studied. This step (Re- 



