72 CONTROL MECHANISMS IN CELLULAR PROCESSES 



the simplest situation would result if isoleucine itself were the in- 

 hibitor. Accordingly, threonine deaminase activity of E. coll extracts 

 was measured in the presence and absence of isoleucine. Table 3-1 

 shows that isoleucine was indeed a powerful inhibitor of the L-threo- 

 nine deaminase of E. coli, whereas the other amino acids showed 

 little or no such activity (Umbarger, 1956). Subsequent study re- 

 vealed that the interaction between threonine and isoleucine was a 

 competitive one and that the enzyme had about a hundredfold 

 greater affinity for the inhibitor, L-isoleucine, than for the substrate, 

 L-threonine (Umbarger and Brown, 1958a). 



TABLE 3-1 

 The Inhibition of Threonine Deamination in Crude Extracts of E. co// 



Amino Acid Tested Concentration Inhibition ( % ) 



L-Isoleucine 10-2 M 100 



L-Isoleucine 10-4 u 52 



L-Leucine 10-2 u 55 



L-Aspartate 10-2 m 30 



L- Valine 10-2 u 



L-Alanine 10-2 m 



L-Methionine 10-2 u 



L-Homoserine 10-2 m 



For conditions, see Umbarger ( 1956). 



Reprinted from Science (Umbarger, 1956) by permission. 



It is clear that the deamination of L-threonine is ideally suited to 

 be the reaction by which isoleucine controls its own biosynthetic 

 pathway. For example, if the utilization of a-ketobutyi'ate (Reac- 

 tions II and Illb, Fig. 3-2) rather than its formation (Reaction I, 

 Fig. 3-2) had been the inhibited reaction, threonine would have 

 been destroyed by deamination to the same extent with as without 

 isoleucine. Also, isoleucine might have inhibited the formation of 

 threonine rather than the deamination. However, inhibition of this 

 reaction would have led to the inability of the cells to grow in media 

 supplemented with isoleucine unless threonine was also present, i.e., 

 isoleucine would have been a growth inhibitor, the action which 

 would have been non-competitively antagonized by threonine In 

 other words. Nature has chosen the only effective reaction for con- 

 trolling isoleucine biosynthesis. 



This mechanism of control can be considered as a negative or 

 inverse feedback mechanism which explains how the oversynthesis 



